To solve the problem of inconsistency of batteries, many people have studied the the battery system with the automatic balancing mechanism which can be regarded as a balanced system. In this paper, a reliability evaluation model for balanced systems with common bus performance sharing considering the degree of balance is proposed. The balance degree of the system is represented by the difference in performance between the components. If the performance difference between components exceeds the balance degree threshold, the system is unbalanced and performance sharing is required to redistribute performance through a common bus. This paper also takes into account transfer capacity limits. When the output performance exceeds the maximum transmission capacity, the performance transmission is performed according to the maximum limit to achieve the best balance. The criterion for system failure is that the balance is still out of balance after performance sharing or the total system performance is lower than the specified requirements. Firstly, by solving the Markov equation, the steady-state probability of component performance levels under the long-term operation of the system is obtained, and then the reliability of the system is evaluated by UGF (Universal generating function) method. Finally, a numerical example is carried out to illustrate the effectiveness of the method.