The influence of semi-ester mass fraction on the properties of epoxy resin was studied, and the better pa-rameters were obtained to prepare the semi-ester modified epoxy resin with excellent performance. Semi-ester modified epoxy resin was synthesized by semi-ester method. The esterification law and the hydrophobicity and thermodynamic properties of the modified resin were studied by acid value calculation, Fourier transform infrared spectroscopy (FT-IR), contact angle (CA) and thermogravimetric analysis (TGA). The change of acid value with time basically conformed to the exponential function. With the increase of the content of semi-ester, the contact angle decreased as a whole, while the contact angle was larger when the mass fraction of semi-ester was 50%, the semi-ester and epoxy resin fully reacted, and the thermal stability was good. The semi-ester modified epoxy resin was successfully synthesized, and the mechanical properties and thermal stability of the modified epoxy resin were improved.