The invention relates to lithium battery pack protective circuits, in particular to a lithium battery pack overdischarge protective circuit of an underwater robot. The lithium battery pack overdischarge protective circuit comprises a reference voltage circuit 1, an input voltage filtration circuit 2, a comparison circuit 3, an underwater sensing circuit 4, a logic control circuit 5, a battery pack positive potential input terminal VIN, a battery pack ground potential input terminal GND, a first relay wiring terminal JO1, a second relay wiring terminal JO2, a conductive probe wiring terminal PB1, a conductive probe wiring terminal PB2 and an external circuit device. Lithium battery pack overdischarge can be protected, and the safety performance of the underwater robot cannot be influenced. The input voltage filtration protective circuit can filter out transient interference, and the anti-interference capacity of the protective circuit can be improved.