With the rotating-arm axis box as the research object, we established a test method in the original place of damage of the axis box that can reproduce the suspension parameter conditions of the bogie frame. First, this method establishes a real axis box mechanical model based on the suspension parameters of the axis box assembly conditions and calculates the load distribution relationship of the axis box through this model. Second, through a comparison of test data under different tooling conditions, it was verified that the distribution relationship had a great influence on the strength evaluation of the axis box body. Then, the single-axis box test method was simulated and optimized to determine the distribution of test loads and constraint conditions. The calculation results better restored the axis box test data under in situ conditions and derived a single-axis box test method with high accuracy. The lowest damage coverage was 1.01. The mechanical modeling and test optimization approach not only improves damage-based life assessment but also tunes up the efficiency of axis box testing.