The invention discloses an offshore platform crude oil riser maintenance and replacement method and device. The device comprises a water proof chamber, a round table sealing assembly, a water pump and water proof chamber pipe clamps, wherein the water proof chamber is of a cavity structure with an upward opening and is provided with a through hole for a crude oil riser to pass through at the bottom, the round table sealing assembly is used for sealing a gap between the crude oil riser passing through the through hole and the water proof chamber, the water pump is connected to the bottom of the water proof chamber through a pipe, and the water proof chamber pipe clamps are respectively connected with the water proof chamber and an offshore platform spud leg. The offshore platform crude oil riser maintenance and replacement method and device utilizes the water proof chamber for the offshore platform crude oil riser maintenance and replacement, thereby reducing the security risk of marine operations, providing safeguard for the safe production of the platform, significantly shortening the platform shutdown period, and greatly reducing the cost of manual output.