The invention relates to an insulation monitoring device of a single-arm unbalanced bridge and belongs to the technical field of power supply detection. The objective of the invention is to provide the insulation monitoring device of the single-arm unbalanced bridge. By setting an insulation monitoring host and a leakage current sensor data acquisition control device, a detection bridge resistor is connected between a negative electrode and the ground, and a corresponding bridge switching test is carried out, so the risk of maloperation of a protection device can be effectively eliminated andinsulation conditions of a direct current system and branches can be calculated. The monitoring device comprises an insulation monitoring host, a leakage current sensor data acquisition control deviceand a leakage current sensor. A first data acquisition module is connected with direct current positive and negative buses, and acquires direct current bus voltage and grounding voltage in real time.A first communication module is connected with a second communication module. A bridge circuit module comprises one group of balanced bridge resistors, corresponding grounding resistors and a detection bridge resistor connected with the direct current negative bus only. The detection bridge resistor is serially connected with an electronic switch.