Today, cloud computing offer IT resources in the form of services. With increasing diversity and complexity of cloud services, a formal modeling method for cloud service interactions is required. In this paper, after introducing basic concept of cloud computing and cloud services, then a formal modeling method of service interaction based on pi-calculus is proposed. Next, a cloud service interaction pattern is given and is modeled by pi-calculus. According to pi-calculus response rules, the interaction pattern is deduced and the result verified its correctness.