The invention discloses a method for preparing a porous membrane. The porous membrane preparing method of the invention comprises the following steps: polymer material is arranged in mixed solvent forbeing mixed, so as to form casting solution; the mixed solvent is of high boiling point solvent with the boiling point being 90 DEG to 300 DEG or low boiling point solvent with the boiling point being50 DEG to 80 DEG; the obtained casting solution is poured on a glass plate or non-woven fabric, and is evenly coated to form a membrane, which is then arranged in the moist air to stand for 10 to 40s econds; the obtained membrane is arranged on gauze in the dry air for 1 to 2 hours; the obtained membrane in the third step is arranged in a drying oven of 90 DEG to 200 DEG or hot blast for 2 to 3hours, until the membrane is completely dried. Compared with the prior art, the method has the following advantages and beneficial effects that a coagulation bath is prevented to be used, the water issaved, and the waste water discharge is avoided; the pore size and the thickness of the membrane are easily controlled, and the membrane performance is easily controlled; and the shrinkage deformation produced in the membrane manufacturing process can be avoided.