Abstract: In order to accelerate the progress of developing the train over-the-horizon alarming system and realize the engineering application of the system as soon as possible, the test plan of the train over-the-horizon alarming system on operating high-speed railway lines over public 5G network was proposed, and the comprehensive overall test of the train over-the-horizon alarming system was carried out on the Beijing-Zhangjiakou high-speed railway. The test comprehensively tested the functions and performance of the train over-the-horizon alarming system, and focused on validating that the vehicle-ground information transmission channel over 5G public network can realize real-time uploading of high-definition surveillance video and safety monitoring information from the ground to on-board monitoring terminals on running trains. Under different line conditions such as plain area, mountain area, tunnel and near station, the functions of real-time video, video on demand and danger warning of the system have achieved the expected effects with desired stability.