At present for the shallow extra low permeability reservoir, displacement efficiency in drainage development is poor and it is difficult for water injection. Whereas, nitrogen gas flooding technology has provided a potential effective means for recovery improvement in extra low permeability area. Combining the field characteristics, through indoor nitrogen displacement experiments in extra low permeability core after a period of water flooding, influence of nitrogen injection parameters on recovery was analyzed. The effects of nitrogen/water alternating injection and pure nitrogen flooding were compared and water/nitrogen flooding slug ratio was optimized. Indoor experiments on natural and artificial cores have proved that nitrogen displacement after water flooding could improve the recovery of 3%-10% and increase the fluid injection pressure, while better gas flooding effect would be achieved under lower water content.There is a new recognition that step rise existed in gas injection amount versus nitrogen gas flooding recovery, which means that gas injection amount can be optimized according to field requirements. Compared with nitrogen flooding, nitrogen/water alternating injection had an obviously increase in recovery and injection pressure that reached the largest value at sixth nitrogen/water alternating injection rounds; in addition, the optimized gas/water slug ratio was 2∶1.