In present, due to continuous increasing of the fracture toughness and continuous reduction of steel plate thickness for ultra-high strength steel used in the aerospace solid rocket motor, it is difficult to measure its fracture toughness of surface crack using the linear elastic fracture mechanics method. In this paper, the tensile test was used to achieve the conditional load when the surface crack initiation occurred. Then, the finite element method (FEM) was conducted to calculate the ductile fracture toughness JIC under the conditional load. Lastly, the surface crack fracture toughness, KIe, can be computed from JIC based on the fracture mechanics method. In addition, the analysis on validity of J-integral showed that KIe converted from JIC was able to evaluate the fracture toughness of ultra-high strength steel. These are important technical parameters for design of aerospace solid rocket motor.