The emission of engines has been a focused issue. With a piston engine as the object of study, Fire was used to establish a type of engine combustion chamber model from the perspective of combustion spray model; changes based on geometric parameters of existing combustion chamber were proposed; and simulation analysis was carried out under the new program; the nitrogen and oxygen compounds and carbon emissions between the old and new program were compared in a transient state. The results show that the emission of nitrogen oxides under the new combustion chamber structure is reduced by 23%, the carbon emission is reduced by 2.8%, and the overall emission is decreased. After installing the modified combustion chamber into the engine and maintaining the engine power and torque unchanged, a reduction in emission is clearly shown, which proved the design program is feasible.