Tank is one of the key components of the reusable rocket， and the integrity evaluation of the weld is difficult for designing and manufacturing of the reusable rocket tank. According to the elastic-plastic principle of fracture mechanics and the two-parameter 2A failure evaluation method， the weld crack of reusable rocket tank is quantitatively evaluated. Based on the evaluation process of tank weld integrity， the method of solving residual strength and critical crack size of tank bottom weld is given. The results show that the critical crack size decreases with the increase of applied stress， and the residual strength decreases with the increasing of critical crack length. Under the same applied stress， the critical crack size decreases with the decrease of the external diameter of the structure. Under a certain critical crack size， the residual strength increases with the increase of the external diameter of the structure. In this study， the failure criteria of welded joints with welding defects for reusable storage tanks are established， which is of great significance to guide the analysis and determination of high-risk failure hazard areas for reusable storage tanks.