As the scale of the software is increasing in the open environment, the structure and architecture of the software is becoming more and more complex and diversified. Using the traditional state-based software reliability evaluation method, the complexity of reliability analysis will be highly increased due to the state space explosion. Besides, traditional reliability evaluation method couldn’t describe system with typical complex structures very well. So, this paper presents an improved state-based reliability evaluation method. It utilizes the user case diagram to divide system and utilizes the sequence diagram to describe the architecture of the subsystems. Both diagrams are used as the input for the software reliability analysis method. This method utilizes a bottom-up way approach to evaluate the software reliability, which adapts to the evaluation of large scale systems with complex structures.