This article deeply explores the theory and practice of smart city planning in the era of big data. The article first introduces the basic concepts of smart cities and their importance in the context of big data, emphasizing the necessity and purpose of smart city planning. Next, through theoretical foundation and literature review, this article analyzes the theoretical foundation of smart cities, the development of big data technology and its application in urban planning, and discusses practical cases of smart city planning at home and abroad. The article further builds the theoretical framework of smart city planning, including the construction of theoretical models, the integration of big data and urban planning, and the analysis of key elements of smart city planning. In the application of big data in smart city planning, this article discusses in detail data collection, processing and analysis, as well as application cases in traffic management, energy optimization, environmental monitoring, etc. Through practical case studies, this paper refines the lessons learned from smart city planning, summarizes the main findings and significance of the research in the conclusion, puts forward policy recommendations based on the research results, and discusses research limitations and future research directions.