The invention provides a maintaining device for a tidal power generation system. The maintaining device for the tidal power generation system comprises a floating type platform, a fixed shaft and a sliding mechanism; the floating type platform floats on the surface of water; the bottom of the floating type platform is connected with the top end of the fixed shaft; the sliding mechanism is connected with a power generation device, under the water, of the tidal power generation system and is used for sliding along the fixed shaft and pulling the power generation device; and when the power generation device needs to be maintained or repaired, the sliding mechanism is utilized to pull upward the power generation device along the fixed shaft to the position near the water surface. According to the maintaining device, by controlling the sliding mechanism to carry the power generation device and move up and down along the fixed shaft, the power generation device to be maintained is transferred from the underwater to the position near the water surface to be maintained, so that the complexity of maintaining work is reduced, the operation difficulty is also lowered, and in addition, the safety hidden dangers of maintenance personnel to carry out underwater maintenance are reduced.