Abstract: Gas migration phase is an important factor influencing gas accumulation and distribution. Gas migration phase was influenced by change of pressure acceptted by organic matter type of gas sourcerock and evolution stage. Gas in water -dissolved phase, in oil -dissolved phase and in diffusive phase can change into gas in free phase. Gas in free phase can change into gas in water -dissolved phase, in oil -dissolved phase and in diffusive phase. Former is favorable for gas to accumulate and form gas pools, Latter is not favorable for gas to accumulate and form gas pool, The change of gas migration phase requires certain geological conditions. Change of gas in water -dissolved phase and in oil -dissolved phase into gas in free phase requires decrease of pressure. Change of gas in diffusive phase into gas in free phase requires concentration caprock or gas -bearing layer.The analysis of actual examples indicates that the gas in free phase released from gas in water -dissolved phase, in oil -dissolved phase and in diffusive phase is the important part for gas to accumulate and form gas pool.