A method for preparing a porous membrane for water treatment, the method comprising at least: 1) mixing a high molecular weight polyethylene, a water-soluble polymer, a pore-forming agent and an antioxidant, and stirring uniformly to form a mixture, wherein the high molecular weight polyethylene has an average molecular weight of 1.0×10 5 to 10.0×10 6 and a density of 0.940 to 0.976 g/cm 3; and the weight of the water-soluble polymer is 5 to 30 parts, the weight of the pore-forming agent is 100 to 500 parts, and the weight of the antioxidant is 0.1 to 10 parts, based on 100 parts of the weight of the high molecular weight polyethylene; wherein the water-soluble polymer is one or more selected from the group consisting of carboxymethyl starch, starch acetate, hydroxymethyl cellulose, carboxymethyl cellulose, polyacrylamide, hydrolyzed polyacrylamide, ethyl cellulose, polymaleic anhydride, polyacrylic acid, polyvinylpyrrolidone, polyvinyl alcohol and polyethylene oxide; and wherein the pore-forming agent is one or more selected from the group consisting of natural mineral oils, C 6-15 alkanes, C 8-15 aliphatic carboxylic acids, C 1-4 alkyl C 8-15 aliphatic carboxylates, C 2-6 halogenated alkanes, phthalates, trimellitates, adipates, sebacates, maleates, benzoates, epoxidized vegetable oils, benzenesulfonamides, phosphotriesters, acetylated monoglyceride, citrate esters and diisononyl cyclohexane-1,2-dicarboxylate. 2) dissolving the high molecular weight polyethylene, the antioxidant and the water-soluble polymer in the pore-forming agent at a temperature to form a mixture, and extruding the mixture by a twin-screw extrusion process; 3) casting the mixture into a strip; 4) extracting the strip to remove the pore-forming agent therein, stretching the strip into a film, subjecting the film to a secondary extraction, and washing; 5) subjecting the film to heat setting and coiling steps to obtain the porous membrane for water treatment. 1) mixing a high molecular weight polyethylene, a water-soluble polymer, a pore-forming agent and an antioxidant, and stirring uniformly to form a mixture, wherein the high molecular weight polyethylene has an average molecular weight of 1.0×10 5 to 10.0×10 6 and a density of 0.940 to 0.976 g/cm 3; and the weight of the water-soluble polymer is 5 to 30 parts, the weight of the pore-forming agent is 100 to 500 parts, and the weight of the antioxidant is 0.1 to 10 parts, based on 100 parts of the weight of the high molecular weight polyethylene; wherein the water-soluble polymer is one or more selected from the group consisting of carboxymethyl starch, starch acetate, hydroxymethyl cellulose, carboxymethyl cellulose, polyacrylamide, hydrolyzed polyacrylamide, ethyl cellulose, polymaleic anhydride, polyacrylic acid, polyvinylpyrrolidone, polyvinyl alcohol and polyethylene oxide; and wherein the pore-forming agent is one or more selected from the group consisting of natural mineral oils, C 6-15 alkanes, C 8-15 aliphatic carboxylic acids, C 1-4 alkyl C 8-15 aliphatic carboxylates, C 2-6 halogenated alkanes, phthalates, trimellitates, adipates, sebacates, maleates, benzoates, epoxidized vegetable oils, benzenesulfonamides, phosphotriesters, acetylated monoglyceride, citrate esters and diisononyl cyclohexane-1,2-dicarboxylate. 2) dissolving the high molecular weight polyethylene, the antioxidant and the water-soluble polymer in the pore-forming agent at a temperature to form a mixture, and extruding the mixture by a twin-screw extrusion process; 3) casting the mixture into a strip; 4) extracting the strip to remove the pore-forming agent therein, stretching the strip into a film, subjecting the film to a secondary extraction, and washing; 5) subjecting the film to heat setting and coiling steps to obtain the porous membrane for water treatment.