In order to guarantee the security of composite insulator as tension string applied in ultra-high voltage （UHV） engineering, it is necessary to study the break impact of insulator string, especially the key factors influencing on impact coefficients. The break test system of composite insulator string was designed and its implementation plan was developed. Test was carried out on two broken insulator strings, the change of tension on insulator strings during test was recorded by dynamic force test system, and the dynamic influence coefficients of insulators under different tension and broken conditions were obtained through calculation. Then, numerical calculation was carried out on broken composite insulator string, and its results were verified with experimental data. Finally, the influences of height and span of junction on impact coefficients were studied with using numerical calculation method, which could provide reference and basis for the further study on the mechanical properties of composite insulator and the development of corresponding design specifications.