In the light of disadvantages of the method of reliability allocation existing, constraints was introduced into the reliability allocation based on weight, and a new reliability optimal allocation method was presented which had both the characteristics of reliability allocation model without constraints and reliability allocation model with constraints of cost. By disusing the feasibility of the method, a corresponding mathematical model was conceived and applied to reliability allocation of airplane system. The result shows that the new method can reflect the characteristics of the two methods and reduce the cost of reliability design of airplane system.