As a kind of new renewable energy, biogas is receiving more and more attentions. However, its combustion duration is longer than diesel, which causes the bigger thermal load of engine. A geometry model of a biogas engine’s piston is set up in Proe 4.0. The temperature fields, thermal stress and thermal deformation of the piston are calculated using ANSYS 10.0. The result shows that the thermal load of biogas engine is still within the thermal strength of the material, thus provides supporting evidence for the popularity of biogas.