Abstract: The reliability of cylinder head bolts in work will badly affect power, dynamic balance and economy performance of the diesel engine. The study put forward a kind of design method combining entropy theory with fuzzy design and optimal design, deduced and built the mathematical model of reliability optimal design for the cylinder head bolts of diesel engine. In this way, it can effectively solve the random and fuzzy problem in mechanical products design. Taking the diesel engine cylinder head bolts for an example which are applied in the mainstream diesel engine of freight transport internal combustion locomotive, through the Matlab programs, the numerical solution can be calculated and figures can be output. The result showed that the weight could be observably reduced when reliability more than 0.9998.