<p>In this paper, an aluminum alloy piston in diesel engine was the research object, the temperature field and stress field under rated working condition were calculated and the effect of explosive pressure was discussed, the accelerated thermal fatigue test and numerical simulation were carried out to study the thermal fatigue behavior, the temperature-modified Manson–Coffin formula was proposed to evaluate the thermal fatigue life, the thermal fatigue failure mechanism was also discussed. The results indicate that thermal fatigue is the main failure mode of piston top, explosive pressure affects the stress of the piston, and the effect is more obvious the closer to the throat. The main crack of the piston occurs at the throat, and its propagation path is irregularly jagged. Under the accelerated thermal fatigue condition, the highest temperature and the maximum stress of the piston appear at the throat. In the heating stage, except for the maximum principal stress, the value of at least one of the other two principal stresses is larger. In the cooling stage, the value of the maximum principal stress far exceeds that of the other two principal stresses. The temperature-modified Manson–Coffin formula was able to evaluate the thermal fatigue life with an error of no more than 2.5%. The difference in the coefficients of thermal expansion of the primary silicon and aluminum matrix is responsible for the thermal fatigue crack initiation in the piston, the crack propagation is a result of the combined effect of the principal stresses at the crack tip and the distribution of the primary silicon, and eventual fracture occurs when the stress exceeds the strength limit of the remaining portion.</p>