In order to study the effect of landfill gas on pore pressure below the leachate level in wet municipal solid waste landfill, a new model for predicting the time and depth dependent development pore pressures due to the formation of landfill gas is derived. Considering the specific flow conditions of pore gas below the leachate level, the model combines the Darcy law, the ideal gas law and the theory of dynamics of fluids in porous media. A finite difference solution is obtained for the liquid-gas migration problem, which is caused by the continuous formation of landfill gas within the wet waste. The calculation results show that the pore pressure below leachate level is mainly influenced by the additional pore pressure caused by the increasing of phreatic surface and the excess pore pressure due to the continuous formation of gas. The pore pressure varied over time during the gas generating period. At the beginning, a maximum gas pressure is identified because of gas eruption, and then, gas escaped from the surface leading to an immediate pressure reduction along with a significantly increased gas permeability; finally, the pore pressure is depended by gas-liquid convection. In the landfill, high gas yield rate, high leachate level and low permeability make waste to form high pore pressure. Drawdown of leachate level is found to be the most effective and workable emergency measure to reduce the pore pressure in wet landfill, which is caused by the continuous formation of landfill gas.