The enhancement between cost and reliability is the developmental direction of modern manufacturing enterprises. On the basis of fuzzy theory, the relationship among the cost of product quality loss, the reliability of the assembly dimension chain and assembly tolerance is studied together in this article. Processing cost can be considerably reduced and the target of quality engineering is realized by optimization design. As an example, a tolerance design model is determined for gear and shaft assembly. Moreover, the mathematical model of the relationship between the cost of fuzzy quality loss and the fuzzy reliability of the assembly dimension chain is determined in combination with a processing cost function. The optimistic results of key dimensions of gear and shaft assembly are identified through through orthogonal experiments. This method can facilitate product quality control by enterprises and the realization of economic targets. The study findings can also serve as references for other similar studies.