The invention discloses an icing online monitoring device. The icing online monitoring device comprises an enclosure, a sliding rail, a tension sensor, a sag monitoring device sensor, a sampling device and an image capturing camera, wherein the enclosure is arranged on a cantilever beam of a power tower through the sliding rail and is formed by a sealed shell and a sensing switch door; a sensing end of the tension sensor is arranged on a stress part of an insulator; a circuit part of the tension sensor is arranged in the enclosure; a data acquisition part of the sag monitoring device sensor is arranged on a power line which is close to the insulator; a circuit part of the sag monitoring device sensor is arranged in the enclosure; the sampling device is arranged on the cantilever beam of the power tower; the sampling device comprises two spring gravimeters, a sample piece guide line which is in the same type of the to-be-tested power line and limiting guide rails; the image capturing camera is arranged in the enclosure and is formed by a fixed base, a telescopic rod, a universal regulating mechanism, a rear semi-circle fixed shell, a front semi-circle transparent telescopic shell and a camera.