In order to investigate the effect of variable valve timing on engine power performance, fuel economy performance and emission performance quantitatively, proceed to optimize it, a high speed diese Specific Fuel Consumption, quantity of NOx and Soot, mass of fresh charge were calculated by 3-D CFD method l engine contains variable valve timing system (VVT) was researched. The effective power, at 6 speeds conditions which valve timing parameters were modulated. A method of calculating CRG was suggested. The results indicate that exhaust advance angle has a little impact on sweep and emission performance, but a big impact on ventilation losses. The optimum exhaust advance angle should be increased as engine speed rises. Increasing intake lag angle in low engine speed could increase coefficient of charge, thereby increase power performance and fuel economy performance. Comprehensive performance could be increased by decreasing intake delay angle under high speed.