Abstract: The forefront of coal seam water injection gas displacement in mathematics is a transition discontinuous interface of gas and water, the seepage process is relatively complex. In order to study the change law of the water saturation on the discontinuous interface, the seepage process of gas displacement by coal seam water injection is simulated with Comsol Multiphysics finite element simulation software. According to the changes of the relative permeability of gas phase and water phase by simulation, the third order TVD format and fourth order TVD format of one dimensional black grid system has been used to calculate the front of gas displacement. The water saturation distribution law of water injection front is obtained that the water saturation decreases rapidly at 0.2 to 0.7,which decreases at first and then maintains a steady trend. This method has a good intermittent capture ability, which has higher resolution for the compressible fluid motion and does not produce false numerical oscillation. The results provide the basis for further study on mechanism of the coal seam water injection to improve gas migration.