The invention relates to a tunnel cross section deformation monitoring device which belongs to the field of tunnel safety and stability monitoring. The tunnel cross section deformation monitoring device comprises a deformed pipe body, an optical cable pipe body, an optical fiber inclinometer, fixed assemblies, an optical cable, a data collector, and a data processor. Take a box-shaped cross section as an example, two ends of the deformed pipe body are fixed to the roof and the side wall of the tunnel respectively by means of the fixing assemblies, and the optical fiber inclinometer is set at aturning point of the deformed pipe body; when the side wall inclines, the turning point of the deformed pipe body is driven to cause an angle change, and the optical fiber inclinometer can detect theangle change of the deformed pipe body; the data collector is connected with the optical fiber inclinometer by means of an optical cable, collects monitoring parameters of the optical fiber inclinometer, and transmits the monitoring parameters to the data processor in a GPS wireless mode, so as to process the monitoring parameters, calculate the inclination amount of the side wall and achieve themonitoring purpose. The tunnel cross section deformation monitoring device solves the problem of inconvenient monitoring of tunnel cross-section deformation in the prior art, and realizes unmanned real-time monitoring.