Time is the physical quantity with the best measurement uncertainty at present, representing the highest level of science and technology. After decades of development, fiber-optic time and frequency synchronization (FOTFS) technology has been widely used in many fields such as quantum metrology, radio interferometry, navigation, modern communication, power grid, high-energy physics, geodesy and so on. It has become a vital support for the efficient operation of human society. Thanks to the high reliability and stability of FOTFS technology, the long-standing contradiction between “distribution” and “real-time” of ensemble time scale has been solved. The paper introduces the development history of FOTFS technology and the research status of each technology path. At the same time, a real-time free-running time scale based on the Beijing FOTFS network is also introduced. Based on this foundation, it is pointed out that the development focus of FOTFS is shifting from technical research to large-scale networked applications. Conducting multi-functional and multi-application integrated research based on optical networks, to achieve integrated communication, sensing, computing, measurement, and control, will be an important development trend in the future.