This paper presents a Petri Net model to analyze the workflow of a web-based multiple participants virtual environment. The presented approach not only can conspicuously help the developer to comprehend the interaction relationship between the client-server virtual environments but also to easily construct a shared virtual world. Based on the presented Petri Net model, we propose an architecture for the collaborative virtual environment and implement a multiple user 3D web browsing system, called the SharedWeb system. Problems of providing the multi-user interaction on the Web and the solutions proposed by the Petri Net model are fully elaborated here. Some experimental results along with two demonstrated virtual world are also presented.