Oil return control method of air conditioning system and three-pipe air conditioning system

The invention provides an oil return control method of an air conditioning system and a three-pipe air conditioning system, and relates to the technical field of air conditioners, if operation parameters meet oil return entering conditions, an oil return process is entered, the air conditioning system is adjusted to be in a pure refrigeration mode for oil return, and a bypass valve communicating a high-pressure air pipe with a low-pressure air pipe is opened; and if the oil return parameter meets the oil return quit condition, quitting the oil return process. Compared with the prior art, the oil return time is reasonably arranged by formulating the oil return entering condition and the oil return exiting condition. A pure refrigeration mode is adopted for oil return, the high-pressure air pipe communicates with the low-pressure air pipe, a refrigerant in the high-pressure air pipe can be discharged into the low-pressure air pipe, the flow speed of the refrigerant is increased, accumulated oil in the low-press