Nowadays, workflow modeling and analysis has become the focus of study with growing demands on business process management (BPM). In this paper, after defining basic elements (task, user, resource and constraint) of business process, then we propose a formal modeling method of unified business process based on the process algebra language-Pi calculus, integrating tasks, users and resources. Besides modeling control flow, interactional behavior of tasks, users and resources, such as task execution and resource access, can be emphasized so that the unified formal model can satisfy actual running requirements. Also, analysis and verification method of model correctness based on process deduction rule of Pi calculus is provided. Next, we establish the Pi calculus model to a concrete business process instance using the modeling approach we propose above. The work reported in this paper provides a theoretical foundation and formal method for modeling and analysis of unified business process.