With the acceleration of urbanization, traffic congestion has become an important factor affecting urban economic development and the quality of life of residents. How to use technological means to improve the transportation system has become an important task in the construction of smart cities. The emergence of digital twin technology provides a shortcut for the development of smart cities. As a current research hotspot, the construction of smart transportation city systems based on digital twins aims to improve traffic efficiency, reduce traffic congestion, enhance urban traffic safety, and provide personalized strategies through intelligent and data-driven methods, thereby improving the travel experience of urban residents. This paper adopts a comprehensive research method, combining literature review, empirical analysis, and case study, to deeply explore the smart transportation city system based on digital twins. At the same time, advanced technological means such as perception devices, data collection technology, digital twin simulation technology, and intelligent analysis algorithms are also utilized to effectively simulate and restore complex traffic scenes, monitor and analyze urban traffic data in real-time, and provide scientific basis for traffic management and decision-making. Build an efficient and intelligent urban transportation system through the integration of a series of technologies and methods. The research results indicate that the application of digital twin technology can achieve comprehensive perception, accurate simulation, and intelligent regulation of urban transportation systems. This not only helps to solve traffic congestion problems, improve transportation efficiency, but also enhances the travel experience of urban residents.