The invention discloses an online monitoring system and method of a full life circle of a railway side slope, and belongs to the technical field of monitoring and early warning of railway side slopes.The monitoring system comprises a data collection and remote control terminal module, a data interface module, a big data center module, an online state monitoring module of full life circle of sideslope, an intelligent diagnosis prediction and early warning module of side slope state and a terminal display module of side slope state, online automatic monitoring is performed on the full life circle of the railway side slope, an environmental monitoring method, a side slope deformation monitoring method and a retaining wall monitoring method are organically combined in the full life circle ofthe side slope to dynamically monitor and intelligently diagnose the state evolution relationship and instability process of the railway side slope in real time from different climates, time periodsand angles, and by adoption of the online monitoring system and method disclosed by the invention, the change rule of the railway side slope can be monitored more comprehensively, accurately and efficiently, and can be intelligently diagnosed and predicted.