The invention discloses an on-site repair machining process of a lower labyrinth ring of a rotating wheel. The process comprises the following steps of firstly, installing a main shaft box, a plane supporting bearing, a lower bearing and a supporting pier on the ground, and installing a jack on the supporting pier; secondly, placing the rotating wheel on the jack, and keeping the rotating wheel parallel to the horizontal plane; thirdly, installing an upper bearing in a water draining cavity of the rotating wheel, installing a main shaft in the rotating wheel, connecting the main shaft to the main shaft box, adjusting the upper bearing, and ensuring that the upper bearing is concentric with the rotating wheel; fourthly, installing a tool arm clamping base, a main tool arm and a balance weight on the main shaft, installing a perpendicular arm on the main tool arm, adjusting the lower bearing, ensuring that the main shaft is perpendicular to the horizontal plane, enabling the main tool arm to be parallel to the horizontal plane, and enabling the perpendicular arm to be perpendicular to the horizontal plane; fifthly, carrying out rough machining, semi-finish machining and finish machining on the position of the lower labyrinth ring; and sixthly, detecting the outer diameter and the surface smoothness of the lower labyrinth ring and evaluating the machining effect. The on-site repair machining process is simple to operate, low in repair cost, short in machining period, high in efficiency and good in effect.