To design a green smart city, which achieves both low carbon and wellness, we propose a new geodesign method using big data. We will highlight the importance of improving walkability in the neighborhood community, as this could contribute to reducing car usage and improve the human health in the city. With the geodesign tool, we can design the future urban systems that integrate building and transportation to improve walkability in the city. This new geodesign approach of integrating big data could play a major role. Big data, such as GPS from mobile phones, would provide useful information to understand people's mobility behavior. It allows us to simulate possible future uses of transportation modes, including car usage and walking behaviors in the city. In this paper, we demonstrate how our new geodesign using the big data approach can be used for our test site case studies in the city center of Tokyo.