Through 1D simulation analysis method, the influences of turbocharging mode on the air exchange process and the performance of diesel engine were studied and the selection principle of optimal valve timing was summarized under the rated conditions of a 6-cylinder diesel engine and a single-cylinder engine with the same bore and stroke. The intake and exhaust conditions of turbocharged 6-cylinder engine and simulated turbocharging singlecylinder engine were different due to different turbocharging modes between them. The difference of optimal valve timing angle between 6cylinder and single-cylinder was obvious by using the same optimization sequence and calculation method and hence the selection principle could be different. The selection principles of optimal IVC and EVO were the maximum charging efficiency and the maximum effective power respectively, which were the same for 6cylinder and single-engines with different angles. The selection principles of optimal IVO for both engines are the maximum actual intake volume and the maximum effective power respectively, and the selection principles of optimal EVC for both engines were the maximum residual gas coefficient and the maximum effective power respectively. Compared with single-cylinder and original engine scheme, the power indexes of power and torque for 6cylinder optimization scheme increased by about 1.5% and 2.6% respectively and the circulating air intake increased by about 1.2% and 3.4% respectively under the rated condition of 2 500 r/min. The validity of simulation method was finally verified.