The effects of valve cone angle α, valve back cone angle β and valve back arc R on intake performance of YN diesel engine with single helical intake port were studied by means of steady flow test and CFD numerical simulation. Then the original valve structure was optimized by orthogonal test method, and the influence of optimization model on in-cylinder gas movement and combustion characteristics was analyzed. The results show that the valve cone angle has the greatest impact on the intake performance, followed by the valve back cone angle and the valve back arc. For the optimization model obtained by the orthogonal experiment (α=35°,β=27°, R=16 mm), the swirl ratio increases significantly with a small increase in the in-cylinder air intake, which can effectively organize the in-cylinder airflow movement, promote the full mixing of fuel and gas, and improve the in-cylinder combustion and emission. In addition, in-cylinder instantaneous heat release rate and cumulative heat release increased,soot emissions reduced, and NO_x emissions increased slightly.