Complex faulted block reservoir is one of the important hydrocarbon resources in China，and it has the characteristics of complex structure，small oil-bearing area and large strata dip. It is difficult to form a perfect injection-production pattern and to produce“attic oil”at the top of the reservoir. As a method for effectively maintaining formation energy and improving displacement efficiency，secondary gas cap drive under huff and puff technique has been proposed and applied to the development of the complex faulted block reservoir. Based on the typical characteristics of the reservoir，the mechanism of the secondary gas cap drive under huff and puff is analyzed，the formation conditions and influencing factors of the secondary gas cap are determined combined with the reservoir engineering method and reservoir numerical simulation，and the forming stage of the secondary gas cap during the nitrogen huff and puff process is studied. The results show that when the gas cap index is greater than or equal to 0.5 during nitrogen huff and puff process in the complex faulted block reservoir，the secondary gas cap with a certain driving energy can be formed. The lower dissolved gas/oil ratio，the lower formation pressure and the larger cumulative gas injection and oil production are more conducive to the development and formation of the secondary gas cap.