The generation of high-precision time-frequency signals and the transmission of ultra-high-precision time-frequency signals are the foundation of modern physics, astronomy, and metrology. The high-precision time-frequency scientific experimental system of the Chinese space station has been successfully launched with the " Mengtian" cabins, which marks a crucial step in the construction of Chinese national time-frequency system with integration and interchange of space and ground. This article provides an overview of the high-precision time-frequency scientific experimental system project and the research progress of microwave time-frequency transmission links, with focus on the signal system design of the microwave transmission link in high-precision time-frequency systems. Based on the proposed signal system, two microwave time-frequency transmission link comparison modes of the same-frequency and three-frequency are designed, and a clock difference comparison model is provided. Finally, simulation and in-orbit measurement data analysis show that the designed microwave time-frequency transmission link can achieve sub-picosecond pseudo-range measurement.