Requirements modeling is a crucial step in the software development process. It takes an important role in requirements engineering. Requirements models are used to discover and clarify the functional and data requirements for software systems. It is the basis for understanding user requirement and designing information system. This paper describes an entire process of building a software requirements model on a Web-based scheduling information system. It firstly makes an informal data analysis. Followed that uses formal methods to design and complete a formal model of the system requirements using the specification language Z. The requirements model is actually formal specifications of the system??s data model, system state and operations. It is a precise and unambiguous mathematic model of the target system. The Z specification of the requirements model can effectively improve system reliability, design time and comprehensibility.