Spark ignition natural gas engines with prechamber, having high thermal efficiency and low exhaust emissions, have been developed due to energy resources and air pollution problems in recent ten years. A lot of significant results on experimental have been given, but the reports on combustion model were few seen so far. This paper discribles a quasidimensional combustion model for the engines with prechamber. A simulation method for calculating turbulent intensity in cylinder and a submodel for jet orifices between two chambers are put forward. It showed by testing that calculated data showed good general agreement with the corresponding experimental values.