Abstract: As a strategic emerging industry in China, the low-altitude economy has achieved large-scale and rapid development relying on diversified low-altitude application scenarios including low-altitude logistics distribution, public services, emergency rescue and other businesses. Nevertheless, traditional standalone ground-only communication, navigation and sensing systems can hardly meet the requirements for full-domain operation in complex low-altitude scenarios. The space-air-ground integrated communication-sensing-navigation (CSN) fusion technology integrates resources including space-based satellites, air-based aerial vehicles, ground base stations and radars, realizing in-depth coupling of three core functions: communication, sensing and navigation. It serves as the core foundation underpinning the safe, efficient and full-domain operation of the low-altitude economy.This paper firstly reviews the development status of the low-altitude economy and core demands of tracking, telemetry and command, and establishes the architecture of space-air-ground integrated information infrastructure as well as the technical system of communication-sensing-navigation fusion. Secondly, from four dimensions covering space-air-ground integrated communication networks, low-altitude communication-sensing integration, communication-navigation fusion enhanced positioning, and situational awareness via multi-source heterogeneous data fusion, this paper systematically summarizes mainstream technical schemes, research achievements and practical application progress at home and abroad. Afterwards, it analyzes common bottlenecks restricting the implementation of communication-sensing-navigation fusion in low-altitude environments, including time-varying channels, multi-service interference, asynchronous data fusion, link security risks and other challenges. Finally, combined with cutting-edge technologies such as 6G, digital twin and industrial large models, this paper predicts future development trends, including ultra-simplified networking, communication- sensing-navigation-computation-control intergration, intelligent sensing, and full-domain management and control empowered by digital twin. This research elaborates the state-of-the-art technical evolution in this field, and provides theoretical references and technical insights for infrastructure construction, technological iteration and engineering implementation of the low-altitude economy.