The work aims to improve the waterproofing, heat resistance and mechanical properties of castor oil based waterborne polyurethane through modification by glycerin monostearate (GMS). The emulsion of castor oil based waterborne polyurethane with different GMS contents was synthesized by acetone method and the emulsion particle size measurement and stability test were carried out, and then the film was made by casting and subject to the tests of surface hardness, mechanical properties, water absorption, contact angle, thermogravimetry, DSC and FTIR. The surface hardness, mechanical properties, water resistance, thermal stability, heat resistance and internal structure of the film were analyzed. The film obtained the best comprehensive performance when the mole fraction of hydroxyl group of GMS was 25%. In this case, the film had a tensile strength of (9.37±1.28)MPa and a Young's modulus of (250.55±34.05)MPa and its water absorption reduced to 12%, and water contact angle was 92°. The addition of GMS increased the Tg and pencil hardness of the film, but decreased the thermal stability. The addition of GMS enhances the waterproofing, heat resistance and mechanical properties of castor oil based waterborne polyurethane.