A regulated two-stage turbocharging system was matched for a 6V diesel engine for the purposes of improving its performance at plateau，as well as solving the problems of turbo surge, overspeed and overheat of exhaust when diesel engine running at high altitude (4.5 km). The simulation model of the regulated two-stage turbocharging diesel engine at different altitudes (0 m and 4.5 km) was set up based on the calibrated original diesel engine model. The influence rule of bypass flow rate of high-pressure stage turbine on the performance parameters of diesel at different altitudes (0 m and 4.5 km) was studied. And the influence mechanism of bypass flow rate of high-pressure stage turbine on fuel economy was analyzed, IMEPg and PMEP were key factors which seriously affected the BSFC when adjusted the bypass flow rate of high-pressure stage turbine. Meanwhile, the influence rule of altitude on IMEPg and PMEP was illuminated, which could be used to explain the influence mechanism of altitude on BSFC of diesel engine. The economic regulated law and reliability regulated law of the turbo system at different altitudes (0 m and 4.5 km) were obtained through the optimization of bypass flow rate. The range of economic regulated law was wider than that of reliability regulated law, and it could observably reduce the BSFC within its regulated area compared with reliability regulated law. At altitude of 4.5 km, the range of economic regulated law was narrower than that at altitude of 0 m. In addition, at medium and high loads, the BSFC was higher than that at altitude of 0 m; however, at low load, the BSFC was lower than that at altitude of 0 m.