The “City in a Park” (CIP) is a new concept of urban transformation and development proposed in China in recent years, guiding the construction of healthy and sustainable living environments. This paper analyzes urban planning based on the CIP concept from a synergetic perspective, aiming to explore how the integrated planning of ecological spaces and built environments can promote systematic sustainable development in ecology, economy, and society. This research employs methods including document collection, unstructured interviews, field observations, and participatory observation, focusing on a case study of the Sichuan Tianfu New Area (STNA), a demonstration zone for the CIP. The study finds that the planning of the STNA extends the planning scope of urban ecological spaces beyond the traditional urban construction boundaries, not only preserving the natural resources but also enhancing the city’s overall sustainability through regional ecological services. By designing ecological spaces as green infrastructure that connects urban and rural areas, the primary sector is more readily integrated with the secondary and tertiary sectors, facilitating the integration of the urban and rural infrastructure and industries. The STNA integrates urban and rural administrative divisions, builds a cross-departmental collaborative management platform, and guides public participation in the planning process, ensuring the efficiency and effectiveness of planning implementation and enhancing the equitable sharing of social services. This research provides new insights into comprehensive, cross-disciplinary, and ecology-oriented urban planning. It offers evidence for an understanding of the application pathways and effects of the CIP concept in urban planning practice and provides valuable experience for other cities to promote harmonious coexistence between the city and nature.