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The historical or legendary accounts of the battle at Shanhaiguan are rather hazy. According to the "History of the Qing," Li Zicheng had an army of over 200,000 soldiers, facing Wu Sangui's 7,000 heavy cavalrymen and 100,000 Manchus. Present-day historians now believe that the Manchus inflated Li Zicheng's numbers in order to glorify their victory, and they believed that in fact Li Zicheng was outnumbered in the fight at Shanhaiguan. Many historians believed that at Shanhaiguan Li Zicheng had 50,000 troops against 60,000 Manchus, Wu Sangui's 40,000 regular and 50,000 militia troops.

Before his conquest of Beijing, it is said that Li Zicheng boasted an army of 60,000 well trained, disciplined, and balanced army of cavalrymen and infantrymen. At that time, his army was divided into five divisions and up to two hundred and thirty companies (lian). Accordingly, each company has 50 cavalrymen, 150 infantrymen, and 30 to 40 support personnel to tend the horses and equipment. This kind of organization would indicate, at least theoretically, a high degree of combination and cooperation between cavalry and infantry. Interestingly, the Shun Jun had a very high ratio of mobile vs. static troops (the ratio of cavalry vs. infantry was 1 to 3, roughly similar to the highly mobile Cold War Soviet Army). It was also believed that Li Zicheng had many cannons in his army, for during his campaigns against the Ming general Zuo Liangyu in Hubei it was said that his cannons and firearms played a pivotal role in defeating General Zuo.

A huge factor contributing to the Shun Army's military successes was its proficient reconnaissance and intelligence-gathering capability. Before he embarked on campaigns against cities or villages, Li Zicheng would send soldiers and spies disguised as local civilians. They were to infiltrate the towns and provide crucial intelligence to Li Zicheng's main force. When the actual attacks or sieges begin, they also have the duty to aid the attackers and serve as fifth-columnists. Similarly, the People's Liberation Army of the Civil War period used these same tactics.

The author believes that the numbers on Li Zicheng's forces on the eve of his confrontation with Wu Sangui were inaccurate. When Li Zicheng built his power-base in Henan, he commanded about 60,000 troops. In addition, in the battles in Henan he faced government forces under Sun Chuanting and Zuo Liangyu, each of whom had forces of around 50,000 men. While it is true that Zuo Liangyu's army was more than a loose group of ill-trained, ill-disciplined bandits, Sun Chuanting's militia was well-trained and posed a grave threat to Li Zicheng's Henan base until he was defeated by Li. When Li Zicheng entered the capital, he had already defeated Zuo Liangyu, Sun Chuanting, Tang Tong, and several other government garrisons, so it is possible to assume that Li Zicheng increased the size of his forces during and after his battles against various Ming armies and militias. Most non-Qing sources estimated that Li Zicheng had between 60,000 to 100,000 men, so a conservative estimate on Li Zicheng's numerical strength was about 80,000 troops (with those extra troops under the command of Tang Tong, Bai Guangen - a former subordinate of Sun Chuanting, and a few other former Ming officers).

After the conquest of Beijing, morale was low among the rank and file of the Shun Army. The soldiers, most of them illiterate farmers who had already looted the riches of the capital city, were yearning to return to their homes in the West. Thus, when they heard rumors of upcoming battles with Wu Sangui at Shanhaiguan, Zuo Liangyu at Hubei, or Zhang Xianzhong at Sichuan, many were unwilling to fight. They believed that they have already rid the country of evil by destroying the old government, so why can't they return home to their loved ones? This belief infested not only among the common soldiers, but among the middle or upper hierarchies as well.

Discipline also broke down once the Shun Army bivouacked in Beijing. Those peasant-bandits, who have never seen the riches and marvels of the capital city before, looted the capital city repeatedly. Li Zicheng, said to be a frugal man, attempted to enforce military discipline and punish the offenders, but there were simply too many criminal offenders to punish or execute. Besides, many of the soldiers' leaders and officers were also greedy bandits, who threatened to rebel against Li Zicheng if he attempts to enforce military discipline.