Gassmann theory is mainly used for dealing with the issue of fluid substitution, but fluid substitution always appears “reverse phenomena” under certain conditions. In this paper, the graphic analysis the cause of saturated rock speed reverse for different rock sample and well log data, found that the density difference of saturated rock is too large causes leading to reverse. At the same time, the saturation of carbonate reservoir rocks were replaced by numerical simulation, the simulation results was consistent with the laboratory test data, only carbonate rocks of different porosity size.