Abstract: The authors take the cooking liquor of Engraulis japonicus as raw material to concentrate its active ingredients with ultrafiltration, and to study the sample pretreatment method andthe relationship among the molecular pore size, operating pressure, flow speed, sample pH andmembrane flux. The results show that the micro-filtration with the pore size of 0.2 m can reduce nutrient loss and completely remove the solid particles. Moreover, the optimal operating conditions of ultrafiltration are determined: pore size 30 kDa, operating pressure 137.90 kPa, flow speed 2.50 mLs-1, sample pH 8.5 and temperature 30 ℃. When the 200 mL cooking liquor is concentrated with volumetric concentration factor of one, the protein concentration factor is 1.61, the protein retention rate is 80.58% and the membrane flux is 94.4 L(m2h)-1.