The invention provides a process for leveling a tower drum foundation ring during foundation construction of an offshore wind turbine. The process comprises the following steps of: fixedly arranging a plurality of horizontal adjusting jacks on the circumference of the surface of back cover concrete according to the diameter size of a transitional section tower drum; fixedly arranging a plurality of vertical adjusting jacks on the circumference of the surface of the back cover concrete; placing a supporting part for supporting the vertical adjusting jacks on the transitional section tower drum; putting the transitional section tower drum down at a corresponding position; measuring the horizontal position of the transitional section tower drum, and pushing the transitional section tower drum to move in the horizontal direction through forth and back telescopic movement of pistons of the horizontal adjusting jacks; after adjusting the horizontal position, fixing the transitional section tower drum at the horizontal position; measuring the verticality of the transitional section tower drum, and changing the verticality of the transitional section tower drum through the corresponding forth and back telescopic movement of pistons of the vertical adjusting jacks; and after adjusting the verticality, locking the vertical position of the transitional section tower drum. By the process,the horizontal position and the verticality of the transitional section tower drum can be adjusted.