Based on pilot fuel spray and mix, combustion chemical reaction kinetic, combustion calculation theory, turbulent flow, NOx formation model, initial and boundary conditions and meshes creation, a three dimension combustion model of a biogas diesel dual-fuel engine was established in this paper. The combustion process in the biogas diesel dual-fuel engine was simulated subsequently. The biogas was made from many kinds of agricultural and forestry residues by a gasifier. The dual-fuel engine was rebuilt by a single cylinder, four-stroke, water cooled, and direct injection diesel engine. The biogas was induced to the cylinder through intake tube of the engine during intake process and ignited by diesel fuel. The simulation results were compared and analyzed with the experimental data of a biogas (made from shavings)-diesel dual fuel engine, and the simulated characteristics of combustion pressure and NOx emissions were in consistency with the experimental ones.