Abstract: Long distance microwave propagation transhorizon over sea surface have been paid much attention in detection and communication and the mechanism analysis and characteristics of observation are significant. In this paper, the mechanism and related principle of long distance microwave transhorizon propagation across seawater are analyzed. The test of C-band and X-band microwave propagation over long distance transmission lines far beyond the horizon over sea surface in September, 2019 is introduced. The propagation principle and mechanism at daytime of the transmission lines with different distances in C-band and 50 km circuit in X-band are summarized and analyzed based on the signal levels record and the observed winds. The research can be candidate for the system design, data analysis and mechanism investigation of microwave long range trans-the-horizon tropospheric propagation projects, especially for the test across sea surface.