Abstract: The clarification of lemon juice was carried out by the combination of ultrasonic-assisted pectinase and ultrafiltration technology. The degree of clarification and pectin content of lemon juice were used as evaluation indexes to discuss the effect of different pretreatment methods on the clarification effect and membrane fouling. The results showed that when a polyacrylonitrile(PAN)ultrafiltration membrane with molecular weight cut off(MWCO)50 kDa was adopt to clarify lemon juice,the influence of different pretreatment methods on the stable ultrafiltration flux was that enzyme plus ultrasonic treatment>enzyme treatment>without enzyme and ultrasonic treatment,the corresponding flux was about 30,33.5 and 13 L·m^-2·h^-1. The optimal operating pressure was determined as 0.1 MPa. Meanwhile,the pectin content of lemon juice was 0.5% and the light transmittance was 99.6%,which indicated the treatment effect reached best. The ultrasonic-assisted pectinase clarifying the juice is conducive to improve the clarity of juice,and in the subsequent membrane treatment process,it has a significant effect on improving the flux of ultrafiltration membrane and reducing the membrane fouling effect.