For the problems of frequent failure of surface collapse and thermal fatigue crack for hot forging mold of NOS525 steel bottom base body during early service, combined with the failure form and location of bottom base body mold, the material composition, hardness and metallographic structure were studied by direct reading spectrometer, Rockwell hardness tester, optical microscope and scanning electron microscope. Then, the sampling design of different depths and directions of the mold was carried out, and the tensile and impact mechanical properties were tested. Furthermore, the forging process of the bottom base body was simulated by software QForm. The test and simulation results show that there are excessively long acicular martensite and a small amount of Weinstein structure in the microstructure of the mold material, which belongs to the superheated structure with coarse grains. The main reason is that the quenching temperature of the mold is too high or the holding time is too long during the heat treatment process, and the tempering is not enough. During the hot forging process of the bottom base body, the temperature of the contact area between working surface of mold and flash edge obviously increases. When the mold is in operation, the maximum stress at the cavity position is close to the yield strength of the material obtained by the experiment, and exceeding the fatigue strength of the material, which is easy to induce surface collapse and thermal fatigue cracks in the above-mentioned parts of the mold, which is consistent with the actual failure of the mold.