Abstract: A back-up power source apparatus in an indoor unit of a multi-split air conditioning system includes first and second voltage detecting circuits and a control circuit connected with the first and second voltage detecting circuits. The first and second voltage detecting circuits detect voltages of first and second power sources on a mainboard of the indoor unit in real time respectively, and the control circuit controls the back-up power source apparatus to enter a corresponding operation state according to the voltages of the first and/or second power sources. A controlling method of the back-up power source apparatus and a multi-split air conditioning system including the back-up power source apparatus are also provided.