The invention discloses an underwater large oil-water replacement oil storing and unloading device suitable for offshore oil development and an operation method thereof. The underwater large oil-waterreplacement oil storing and unloading device comprises a deck, an undersea oil storage bin, an oil feeding pipeline, a replacement water tank, a water storage pump, a ballast water tank, a ballast water pump and an oil discharging pipeline; the oil feeding pipeline is positioned on the deck; the top of the replacement water tank communicates with the atmosphere, and the bottom of the replacementwater tank selectively communicates with the bottom of the undersea storage bin through a water feeding pipeline; an outlet of the water storage pump communicates with the replacement water tank; thebottom of the ballast water tank communicates with the bottom of the undersea storage bin through the water feeding pipeline; and the top end of an outlet of the oil discharging pipeline is lower thana first overflow opening in the replacement water tank. According to the underwater large oil-water replacement oil storing and unloading device suitable for offshore oil development and the operation method thereof, crude oil underwater storage operation and unloading operation are achieved by cleverly utilizing the static pressure water head difference without any oil pump conveying equipment,and thus the complexity of a system is lowered.