Based on GT-POWER software, simulation model of working process of a long-stroke sequential turbocharging (STC) diesel engine is set up, the calculated result tends to be in agreement with the experimental ones. It proves that the calculation model,method and the selection of the relative parameters are reasonable and the model could predict and improve the performance of the diesel well. To avoid the surging problem and extend its working area, performance parameters of diesel which uses STC with bypass are calculated and compared with experimental data. The paper mainly analyzes the performance and running process of diesel. Finally, the switch point and bypass area of STC are confirmed.