Abstract: The PU/SiO2 composite ultrafiltration membranes with different content and diameter of SiO2 were prepared based on wet phase transition method. The properties and structures of the membrane were investigated via membrane instrument and SEM. The results showed that with the adding of SiO2 the pure water flux of the composite membrane can be improved acutely. The effect of SiO2 with different diameter is not the same. The retention and porosity of the micrometer SiO2 composite membrane increased firstly then decreased when the percent of SiO2 is too high, but the effect and the dosage were different because of the diameter of SiO2. The ultrafiltration membrane was used to treat the black paper-making liquor and gained good results.