Abstract: Based on the linear elasticity fracture mechanics, the finite element analysis was applied to predict the fatigue crack expansion of the lifter in the valve s eat of emulsion pump.A I + I mode (open type+slip type) equivalent stress strength factor of the mixed type cracks was obtained and was applied to a Paris formula, t hus the crack expansion value and the total crack length was calculated and obtained.The mixed type crack expansion orientation angle also could be calculated with I type (open type) and I type (slip type) crack stress strength factor.The analysis showed that before the crack expanded, the finite element crack expansion orientati on angle would be 45° to the spiral shaft of the lifter and would be high fitted with the experiment results.