The invention relates to a combustible gas explosion decompression area determination method for 220kV transformer substation main transformer room, and belongs to the safety application technical field of transformers. The combustible gas explosion decompression area determination method for 220kV transformer substation main transformer room firstly selects maximum allowable explosion overpressure of the 220kV transformer substation main transformer room according to the requirement of construction work side or design side, secondly calculates the decompression ratio of the main transformer room according to the selected overpressure, and thirdly calculates the explosion decompression area according to the decompression ratio and the indoor volume of the main transformer room, thus the combustible gas explosion decompression area of the oil immersed transformer substation main transformer room can be determined. The decompression area is calculated according to the maximum allowable explosion overpressure for avoiding the problem that the determination to the decompression area is inaccurate due to the fact that the decompression ratio is selected according to the category of the stored explosive of the prior art, and the combustible gas explosion decompression area of the 220kV transformer substation main transformer room can be accurately determined.