Abstract: For the XHA9 medical linear accelerator, the measuremed depth-dose distribution in a water phantom shows some defects including less-than-desired nominal electron energy and higher relative surface dose. In order to solve these, the initial electron model of XHA9 was established in model E7 by Monte-Carlo code BEAM. The percentage depth-dose distribution curve of model E7 in water phantom was calculated and proved to be nearly the same as the measured one, which indicates that structural optimization of the treatment head is necessary. Contributions of each component to the therapy electrons determine that the main contribution is the large amount of low-energy electrons scattered by the applicator. Structural optimization of the treatment head was performed by modifying the applicator and the scattering foils. Final simulation results prove that the optimized scheme can enhance the nominal electron energy, decrease the surface dose and the bremsstrahlung dose, and ameliorate the dose uniformity.