When the three times oil and gas recovery system detects that the pressure in the tank reaches the preset start-up pressure (such as +150Pa), the system starts to work; the compressor extracts oil and gas from the tank and compresses it, the elevated temperature oil and gas passes through the condensing device for room temperature cooling, and part of the oil and gas is condensed directly into liquid oil; the remaining oil and gas/air mixture continues to enter the membrane module with selective permeation function The remaining oil/air mixture continues to enter the membrane module with selective permeation function for separation, and the gas mixture is divided into oil-rich permeate phase - supersaturated oil and gas and purified air, which is pumped back to the tank by vacuum pump, while the purified air can be discharged into the atmosphere directly. With the operation of the oil and gas recovery system, the pressure above the tank will gradually drop, when the pressure at the top of the tank drops below the stopping pressure (e.g. -150Pa) or when the equipment stops for a certain period of time, the system will automatically stop until the pressure at the top of the tank rises again and reaches the set start-up pressure, then the equipment will start again.