In response to Hong Kong's smart city development strategy, this paper takes the 3D digital map construction project in Kowloon as a practical case study and systematically presents a construction method for 3D digital mapping based on multi-source data fusion. Aiming at the technical challenges in high-density urban environments-including dense buildings, complex 3D traffic networks, and severe shadow occlusion-an "air-ground fusion" data acquisition strategy is proposed. By comprehensively adopting multiple approaches such as oblique aerial photography, Vehicle Mobile Mapping System (VMMS), and Portable Mobile Mapping Survey (PMMS), a high-precision and highly realistic urban 3D model has been constructed. The paper focuses on the principles of multi-source data fusion based on feature registration and combined adjustment, as well as the 3D modeling process and the quality control methods for the final results. The project’s technical innovation and practical feasibility have been validated through international benchmarking. The research results have been applied to urban planning, traffic management, environmental studies and other fields, providing a solid data foundation and technical support for Hong Kong's smart city development.