The invention discloses a device and a method for detecting canal lining concrete diseases and lower part media under the water. The device comprises an on-water control platform, an under-water crawler type traction vehicle, an illumination system, an under-water high-definition image collection system, a geological radar host and a radar antenna, wherein the radar antenna is charged into a sealed pressure-resistant transparent protection case. The control of the under-water part is realized by the on-water control platform on the embankment or a boat; through the comprehensive detection on optical imaging and geological radar electromagnetic waves is performed by the illumination system, the under-water high-definition image collection system and the radar antenna on the canal under-water lining concrete diseases and lining lower part hollowing, filling and building quantity, underground water conditions and the like. The problem of geological radar under-water detection difficulty is solved; the comprehensive detection of optical detection and geological radar multivariate information can be realized; the device and the method belong to the effective and fast detection device and method for under-water concrete building self and lower part various medium conditions.