The multi-phases assembling method is proposed to calculate the true stress - true strain relation for plastic in standard tensile test. By this method, the measured load-elongation curve is divided into three phases. According to characteristic of each phase, the true stress-true strain relation is obtained by distinct analyzing method. The true stress-true strain curve for the whole test is extracted from the data of load-elongation record and assembled. Results from experiments of four plastics verify the efficiency of this method.