The invention discloses an oil-gas displacement process for an underwater oil storage tank, which comprises the following steps of: 1, in an empty state, displacing all air in the oil storage tank into nitrogen by using nitrogen; 2, when oil is fed, delivering the crude oil into the underwater oil storage tank, and discharging the nitrogen till the crude oil in the oil storage tank reaches the specified height; 3, when the oil is discharged, opening the valve door of an outward delivery control valve, starting a crude oil lift pump, and delivering the crude oil outwards through an oil delivery pipe; meanwhile, after the nitrogen pressure in the oil storage tank is reduced to the specified pressure, opening a pressure control valve so that the nitrogen enters the underwater oil storage tank, and when the liquid level in the oil storage tank is reduced to the specified height, closing the crude oil lift pump to stop outward delivery; and 4, performing outer circular heating, namely whenthe average temperature of the crude oil in the oil storage tank 18 is reduced to the specified temperature is detected, starting the outer circular heating flow. The process is simple, avoids environmental pollution and oil product loss due to the oil mixing problem, and meanwhile, avoids heat loss because oil and water are directly contacted.