In this paper, based on LJ465Q as the original model, designed 3 different lengths of intake manifold according to the effect of gas in the manifold with the principle that the diameter and curvature of the case must be same. A three-dimensional numerical simulation of 3 different lengths of the intake manifold was done by Using FLUENT software, and the gas flow of the manifold was analyzed, the best length of the intake manifold obtained was obtained. The results showed that the length of the intake manifold has large effects on the volumetric efficiency, multidimensional numerical simulation is a very effective means in the internal combustion engine development.