The invention relates to a method for establishing and analyzing an online monitoring system for a 500kv power transmission tower in a mountain environment. According to the method, the main control factors influencing the stability of the power transmission tower are determined to be geological disasters and wind loads with the 500kv power transmission tower with geological hidden dangers as the research core, and the influences include tower footing displacement and tower body vibration; the online monitoring system is determined to comprise a tower footing monitoring device, a tower body monitoring device, a monitoring data remote transmission device and an early warning device with the main control factor influencing the stability of the tower as the standard; based on tower footing deformation-time sequence, a tower footing three-dimensional numerical model based on a local reduction method is established, tower footing safety factors are determined, and tower footing stability is evaluated; based on the real-time monitoring data of the online monitoring system, relative deformation between a tower body and a tower footing is calculated, a tower deformation-time sequence database is formed, a grey correlation model is established, the tower deformation development trend is predicted, and the health status of the tower is evaluated.