Taking ±800 kV DC composite arrester of ultra-high voltage （UHV） Xinsong converter station as the research object，finite element models of arresters with additional guy cable are established. The analysis of the dynamic characteristics and the influence of initial prestress are carried out.The response of acceleration，displacement and stress of the arrester before and after setting guy cable are calculated and analyzed. The results show that the horizontal stiffness of the arrester can be improved by setting guy cable，and the natural frequency of the arrester can be increased. The larger the initial prestress of the guy cable is，the more obvious the fundamental frequency of the arrester will be. Increasing the initial prestress of guy cable can effectively reduce the seismic response of arrester. When the initial prestress of the guy cable is 100~150 MPa，the seismic response of arrester decreases most obviously； When the initial prestress of the guy cable reaches 200 MPa，the stress response of the root of bushings of the arrester can be greatly reduced by setting guy cable. The safety factor of bushing can be increased from 3.76 to 6.15，and the top displacement response of arrester under earthquake can be reduced by 50.2%. The effect of seismic reinforcement is significant，and keeping on increasing the initial prestress of the guy cable to higher than 200 MPa has little effect on the seismic capacity of arrester.