The invention discloses an underwater object monitoring device. The underwater object monitoring device comprises an afloat platform, a sonar system, a signal processor, a positioning processor, an alarming processor and a data transmission unit, wherein the afloat platform can float below water surface and at a preset depth. A sonar array sends out acoustic signals; the sonar processor receives the echo signals of detection signals and acquires the locating information and characteristic information of an object; the signal processor realizes monitoring according to the locating information and the characteristic information. Therefore, as long as the object is within the detectable range of the acoustic signals, the object can be detected and found out, monitoring according to visual effects is not needed, and adverse impacts on monitoring effects, caused by environmental factors, can be avoided. The data transmission unit reports the locating information of the object and the position information of the underwater object monitoring device, so as to help monitoring personnel to find out whether the potential invading object exists or not and find out the position of the potential invading object. Therefore, the underwater object monitoring device can effectively monitor underwater conditions.