Abstract: For the coupled mechanical-gas settlement model and calculation for MSW by considering biodegradation, two aspects are presented here: biodegradation laws and coupling effects. A new model to express the decomposition is proposed by analyzing the data of biodegradation experiments. The model for the coupled mechanical-gas effects combines the Darcy law, the Fick law, the ideal gas law, the principles of effective stress and the theory of dynamics of fluids in porous media. The coupling effects and gas movement are linked by the unsaturated gas constitutive theory. The Gibson and Lo's model is adopted as the constitutive model. The proposed coupled model with biodegradation settlement can reflect the rules of total settlement and gas pressure. The results show that the settlement can reach 16.7% of the initial height for a constant load of 9.5 kPa. Because of the biodegradation, gas pressure will increase at the beginning to the top value, and then decrease. The proposed model can fit the experimental data very well.