Abstract: At present, nitrogen flooding technique is widely used in the Tahe oil field, leading to significant oil increment. However, the effect of nitrogen flooding in well groups is different, and the influencing factors are not clear. The karst background was divided into three types, including weathered crust+subsurface stream composite karst, subsurface stream karst and broken solution karst, based on the understanding of fractured-vuggy reservoirs. The influences of reservoir depth, remaining geological reserves and injected gas-water ratio on the effect of nitrogen injection in well groups were studied. The influencing factors of nitrogen injection effect under different karst backgrounds were analyzed from 35 injection well groups that had completed evaluation, so as to guide nitrogen injection works in oil fields in the future.