CO2 huff and puff has many advantages in developing low permeability reservoirs,such as small investment and quick return. Particularly ,it has the same efficiency with hydraulic fracturing in developing complex low permeability fault block reservoirs. However, there are many factors influencing the efficiency of CO2 huff and puff in developing low permeability reservoirs.Soak time is one of these important parameters.According to the phenomenion that there exists optimal soak time in the experiment,this paper studies the cause of optimal soak time from the relative position of CO2 mobile front and pressure drop front,that is due to the difference of their relative positions, recovery efficiency and oil-draining rate are different,eventually produces the optimal soak time.The determination of optimal soak time plays an important role in improving the development of low permeability reservoirs.