In order to effectively utilize cotton pulp black liquor and reduce environmental pollution，the black liquor-phenolic resin composite gel was prepared by hydrothermal method with alkaline cotton pulp black liquor，phenol，formaldehyde and polyacrylamide as raw materials. The gel was characterized by FT-IR，TG-DTG and SEM. The effect of phenol addition on the gel was studied，and the salt tolerance，acid and alkali resistance and blocking effect of the gel were evaluated. The results showed that the network of black liquor- phenolic resin gel was compact and the thermal decomposition temperature was 200℃. The gelation time of the gel with 2%—4% phenol addition was 24—14 h，the moisture content was 72%—75%，and the water absorption ratio was 3.60% —3.92% . The gel had the advantages of low viscosity，easy pumping，good salt tolerance，and acid and alkaline resistance. The sealing effect of gel was good. The sealing rate of artificial simulated core was more than 99% . Preparation of plugging agent by black liquor from cotton fleece could not only improve the pollution of black liquor，but also reduce the exploitation cost of heavy oil.