Nowadays in China, as the economic reform goes further and the central government increasingly demands for raising productivity, more and more state-owned factories begin to turn their heads back to problems arising from their production systems. With the co-operation of Tianjin Diesel Engine Factory, we conducted a simulation study on its diesel engine assembly line, using GPSS as our major simulation language tool. This paper describes the model we constructed, simulation experiments we made on the model, and some conclusions we drew from the simulation study.