The utility model provides a small -size ships and light boats bow door dropout alarm device belongs to ship domain, solves small -size ships and light boats bow door in case dropout and manipulation personnel in time find the problem of harm navigation safety, and this alarm device is divided by detector circuit, controller circuit, alarm circuit and the power supply circuit four bibliographic categories and forms, power supply circuit communicates the back, when the locking device of ships and light boats bow door when detaining, the detector closing of circuit leads to controller circuit to be off -state, unable signal output to the alarm circuit that produces, the alarm is out of work, no buzzing warning, when the locking device of ships and light boats bow door threaded off, the disconnection of detector circuit leaded to controller circuit to be the route state, and the alarm circuit is inputed to the producing signals, and alarm work takes place the buzzing and reports to the police, reminded the manipulation personnel in time to deal with the dropout problem, guaranteed the navigation safety.