War-broken parts could be repaired quickly without power and gas in the field by using the thermit welding technology. In this paper, experimental and structural analysis on strength of thermit welding was made. According to orthogonal experiments, the pressure mode was found to influence tensile stress most, then the warm-up temperature and the welding line clearance. By quadratic general revolving combination design, the mathematics equation indicating the relation of welding tensile stress, warm-up temperature, pressing time and welding line clearance was obtained, and the optimized welding parameters to achieve the best tensile strength were decided as well. By further analysis on microstructures and fractures of welded seam, fusion zone and HAZ of thermit steel, conclusion was made that it was of vital significance to adopt welding-after-peening method for improving the strength of the welded seam.