With the intensification of urbanization, the application of contemporary technology to make cities smarter is the key to their sustainable development (SD). This study aims to propose a comprehensive assessment framework for the SD of smart cities. First, an assessment system with 5 dimensions and 25 indicators is proposed in this paper. Second, a Z fuzzy-based multiple criteria decision-making (MCDM) model is developed to clarify the internal influence of the indicators and to determine the SD performance of smart cities. The Z-DEMATEL (decision-making trial and evaluation laboratory) technique was used to determine the mutual influence relationship of the indicator and their influence weights. Moreover, this paper selected one well-known city in China as a case study and used the Z-TOPSIS-AL (technique for order preference by similarity to ideal solution based on aspiration level) approach for analysis. The results demonstrate that quality of life, per capita GDP, and GDP growth rate are the top three indicators, which means that decision-makers should pay more attention to these indicators when constructing and managing a smart city. This study provides a reference for follow-up related research, and the management findings provide a basis for managers to make decisions on the development of smart cities.