Abstract: Matlab optimization function was used to improve optimization conditions of representative stress-stain method and optimize calculation flow of representative stress-true stain method, and yield strength σ_y, strain hardening exponent n and elastic modulus E three parameters were simultaneously optimized by using improved elastoplastic equation. Thus the mechanical property parameters were determined. Then the mechanical properties of P91 steel were tested by ball indentation representative stress-strain method and compared with uniaxial tensile test results. The results show that compared with uniaxial tensile test results, the mechanical properties of P91 tested by ball indentation representative stress-stain method had high accuracy and the maximum deviation was 12.11%.