Cavitations is the major problems of designing water hydraulic valve, understanding the relationship between shape of valve and cavitations , avoiding cavitations is extremely important, this paper, the numerical simulation and visualization research of flow field is carried out to research relation between the flow field of poppet valve and the length of valve seat by finite volume numerical method. It is some useful conclusions: the negative pressure of the connecting place of cone and cylinder will produce relatively large effects with changing of length of valve seat and chamfer