A quasidimensional combustion model for a dualfuel engine was developed based on a doublezone fuel spray entrainment model, which uses fractal theory to describe the geometry of flamelets in turbulent combustion and its propagation as well as its emission performance. The doublezone fuel spray entrainment model is consisted of ignition delay model, fuel spray and air entrainment model, and flame propagation model. Subsequently, the simulation for combustion processes in a biogas diesel dualfuel engine was conducted on to forecast its Nox emission characteristics. The relative error between the simulated results and experimental data is no more than 5%. The Nox emission of the biogas diesel dualfuel engine increases with the increase of pilot fuel quantity and the advancement of fuel delivery advance angle.  