Mathematic model of the working process for new type hypocycloid single-cylinder diesel engine was built base on thermodynamic theory. And simulation for the pressure and temperature in cylinder, the mean effective pressure and so on, was performed in Matlab software. The results show that the model exactly predicts the performance of the new type hypocycloid single-cylinder diesel engine. The new engine has a higher cylinder pressure than the conventional S195 diesel engine. The maximum pressure and indicated mean effective pressure is improved by 5% and 10.7%, respectively. And the transformation mechanism of the new hypocycloid energy improves working process of the engine.