Poor reliability of engine block is a technical problem in airworthiness certification of aviation piston engine. Taking the engine block of a certain type of engine as the research object, the transient load curve of the engine block is obtained and matched through the model, and it is restrained according to the actual installation. The Goodman method is used to analyze the high cycle fatigue of the engine block, the fatigue safety factor of block is obtained. A design method of separated main bearing cap is put forward to optimize the structure of the lower engine block, and the comparative analysis is carried out under the same load conditions. The analysis results show that the fatigue safety factor of the lower block and engine life are improved, and the reliability index is verified by Weibull mathematical model. The durability test of the optimized engine block shows that the reliability of the engine block is improved, which proves that the design method is feasible, and it can be used for reference to improve the reliability of similar structures.