The invention discloses a method and a system for controlling oil returning of a cold and hot water air-conditioning unit. The method for controlling oil returning of the cold and hot water air-conditioning unit comprises the following steps: when an oil returning instruction is received, controlling the cold and hot water air-conditioning unit to be in an oil returning mode, and monitoring the water temperature of the cold and hot water air-conditioning unit in real time; judging an operation mode of the cold and hot water air-conditioning unit; when the cold and hot water air-conditioning unit is in a refrigeration mode, comparing the water temperature monitored in real time with a preset low temperature threshold value, and when the water temperature is less than the low temperature threshold value, controlling the cold and hot water air-conditioning unit to stop the oil returning mode; and when the cold and hot water air-conditioning unit is in a heating mode, comparing the water temperature monitored in real time with the preset high temperature threshold value, and when the water temperature is greater than the high temperature threshold value, controlling the cold and hot water air-conditioning unit to stop the oil returning mode. According to the technical scheme, the phenomenon that a water heating heat exchanger is frozen can be avoided when the cold and hot water air-conditioning unit performs refrigeration, and the phenomenon of high pressure protection caused when the cold and hot water air-conditioning unit performs heating can be avoided.