Its difficult get a detailed analysis of the Han military since western scholarship hasn't examined Ancient Chinese history as indepth as western ancient history, but i will try to explain how the Roman military system and Han Chinese military system differ. It comes down to military scale, Tactics and strategies, and professionalization.

Military Scale

The roman legion at it's height was 250 000 roman legionaries under Severnus, composing 60% of the size of the total roman military in the 3rd century. It is important to note that the crisis in the third century was caused partially by an extremely large and expensive standing army, so its likely that 250 000 standing legionaries was unsustainable. During the period of Augustus, roman legionaries were 125 000 (40%). This means that the standing army of the roman empire was between 300 000 to 450 000 soldiers during the imperial age, including auxilia that were lesser in quality.

http://en.m.wikipedia.org/wiki/Roman_army#Imperial_Roman_Army_.2830_BC_.E2.80.93_AD_284_.29

Han China military dwarfs Rome's military in size. During the Han-Xiongnu war, Han military size was recorded to be 400 000 infantry and 80 000- 100 000 cavalry at the beginning of the war, and growing to 700 000 infantry and 200 000- 250 000 cavalry. This dwarfs the roman army even at it's largest extent.

http://en.m.wikipedia.org/wiki/Han–Xiongnu_War#Military

Professionalization

Roman legions were professional, career soldiers. Probably one of the best trained and disciplined in the world at the time. But its important to note that for most of rome's history, the legions were numerically inferior to the auxilia, which were recruited from non romans and were not as well trained.

China's Han dynasty was largly conscripts, militiamen who are lower in quality. However, it is likely that the Cavalry were professionals or at least very well trained given their utilization as shock troops during the han xiongnu war. Again, its difficult to really know how Han structured their military without knowing Chinese.

Military tactics and strategem

Roman military doctrine was essentially seeking out direct confrontation when it is possible, if not force the enemy to confront them by targeting population centers or supply stores. This is due to the military superiority the romans enjoyed over their enemies, they can afford to fight in this manner. Some roman generals took the Chinese approach (like Caesar) but they were the exception.

In contrast, Chinese military doctrine is very much like Sun Tzu's art of war: indirect approach, diplomatic or political isolation over military victory, and only seeking battle when you are ensured victory. The chinese would have no problem waging a guerilla war, or avoiding direct confrontation if it suits their stratic goals. Whereas the romans saw this as a dishonorable way to conduct warfare.

I hope this helps, i'm much more well versed in roman history than chinese, but i tried to explain the differences as best i can.