The invention provides a multi-bus grounding insulation monitoring method in three buses and a monitoring device. The monitoring method comprises the following steps of setting a constant voltage source between a reference point and ground; and detecting the actual voltages of each bus to the reference point and the sum of the leakage currents of all the buses when the constant voltage source is set to be a positive set value, 0V and a negative set value respectively, and calculating the resistance of each bus to the ground according to the actual voltages of each bus to the reference point and the sum of the leakage currents of all the buses when the constant voltage source is set to be the positive set value, the 0V and the negative set value respectively. In the technical scheme, the resistance of each bus to the ground is calculated according to the actual voltages of each bus to the reference point and the sum of the leakage currents of all the buses when the constant voltage source is set to be the positive set value, the 0V and the negative set value respectively, parameter measurement and a calculation method are simple, and a problem that the insulation monitoring method in the prior art can not be used to monitor the insulation condition when the multiple buses in the three buses of the positive bus, the negative bus and the zero bus are grounded can be solved.