Abstract: With soybean oil and liquid paraffin as mixed diluent, polypropylene (PP) waterproof breathable membrane was prepared by thermally induced phase separation (TIPS). The morphology and properties of the membrane were characterized by scanning electron microscopy (SEM) and water and gas flux measurement. The results show that the addition of liquid paraffin to soybean oil can effectively increase the porosity of the membrane and greatly improve the waterproof and breathable properties of the membrane.