Abstract: Chang 2 reservoir of Yanchang formation is a main reservoir in Mesozoic of Ordos Basin. Previous scholars have studied the main controlling factors of Chang 2 reservoir in many aspects, while there exist two shortcomings in their studies. On one hand, their studies on the controlling factors were not combined with the upper and lower strata.On the other hand, the reasons for Chang 2 reservoir accumulation were not clear, which experienced long distance lateral migration and there is lacking of fault studies. Based on the previous studies, it is considered that the caprock or a sheltering condition is one of the main controlling factors for hydrocarbon accumulation, but it is difficult to define whether a certain formation is an effective caprock or a sheltering condition. It should be considered that there may be small faults in Yanchang formation, rather than that the faults in the basin are not developed, just as many scholars once said.