The invention relates to a method for testing mechanical properties of an alloy forging and a stress relaxation value simulation method, which belong to the technical field of mechanical properties test, and solve the problems of difficult mechanical properties and long experiment time of a material obtained by tempering value simulation in the prior art. The testing method comprises the following steps: performing tempering thermal treatment on a single tissue sample at several quenching states, and under different temperatures during a heating process, taking the samples and cooling the samples to obtain a heating sample; sampling at different times during an insulation process, and cooling to obtain an insulation sample; sampling at different times during a cooling process, and cooling to obtain a cooling sample. The heating sample, the insulation sample and the cooling sample are heated to the sampling temperature, and after the temperature is uniform, a stress strain test is carried out to obtain the mechanical property parameters of the alloy forging. The testing method can be used for testing the mechanical properties of the alloy forging.