The invention relates to the technique of underwater motor power supply loop detecting and protecting, in particular to an insulation monitoring and protecting control device and a method for a multichannel underwater motor power supply loop. An insulation monitor is connected with an insulation detecting switching device, the insulation detecting switching device is connected with a plurality of underwater motor power supply loops, and the insulation monitor is formed by the connection of an insulation detecting switching circuit and an insulation protecting circuit through a microprogrammed control unit (MCU). The underwater motor power supply loop comprises a motor, a power supply, a control motor and a shunt excitation coil used for the opening or closing of the power supply. By means of the insulation monitoring and protecting control device and the method for the multichannel underwater motor power supply loop, the usage cost is cut down greatly, and a lot of installation space is saved. MCU control is adopted by the insulation detecting switching circuit, the expandability is strong, a user is enabled to write programs to expand the system function, and thereby the expandability is further strengthened, and the insulation protecting function is enhanced.