A modular-based Green Design Studio (GDS) platform has been developed in this study for fast and accurate performance analysis for early stage green building design. The GDS platform aims to simplify the design and analysis process by embedding performance parameters into design elements in modules and employing near-real-time model for whole building performance simulation as well as by providing an easy-to-use and intuitive user interface to assist users without extensive knowledge on building physics. The platform consists of building modules as fundamental building blocks, performance predicting models, and a user interface for visualization and interactive design. In the platform, a whole building is composed of modules organized in a hierarchical structure, including spaces, enclosures, service systems, sustainable resource systems and sites. Both physics-based and data-driven models can be used to simulate the building performance and optimize building systems. A simplified physics-based model, the Resistance- Capacitance (RC) model, has been proposed as a generic simulation model for the flows of heat, air, moisture and pollutants, which is significantly faster than conventional simulation tools such as EnergyPlus, and hence more practical for use in real-time design interaction and optimization. A pilot case study is conducted to illustrate the modular-based design approach using a section of an office building. Compared to conventional building performance analysis tools, the GDS platform can provide fast and reliable feedback on performance prediction for early design. The modular approach makes it easier to modify the building design and evaluate the potentials and contributions of various green design features and technologies.