Referring to the sealing specific pressure of gasket and valve seat, a new valve leakage evaluation method was built in perspective of mechanics, and a new valve leakage evaluation model was built, which realized the quantitative analysis and evaluation of valve leakage. The effect of valve seat dimension on valve seal performance was analyzed by the example of filling valve. The result reveals that valve seal performance is better with less valve seat radius under the premise of valve nonmetal sealed material not breaking.