Automatic pile body stress test system is used in an in-situ static loading test. It’s found that strain process line has a step-like feature. When unloading, as the rebound amount of soil is less than the amount of pile body, negative frictional resistance exists in the soil. It’s considered that concrete damage model can explain the modulus “reduced” phenomenon of calibration section’s concrete. Based on damage mechanics theory, a function of modulus of concrete and its own strain are obtained. According to actual strain of pile body, different modulus are adopted to calculate pile body stress. The in-situ test results show that this method is more reasonable.