The utility model relates to an underwater radial distance measurement apparatus for reactor internal, wherein the measurement apparatus can precisely measure a radial distance between two surfaces of the reactor internal underwater. The underwater radial distance measurement apparatus comprises a probe (1) and a position feedback cylinder (2); the probe (1) can be pulled back and forth and is connected with the left end of the feedback cylinder (2); and the feedback cylinder (2) has a cylindrical structure; and a left air supply pipe (7) and a right air supply pipe (8) are respectively arranged at the left end and the right end of the feedback cylinder (2). A left rack (9) and a right rack (3) enable the position feedback cylinder (2) to be connected with a mounting base plate (4) from the left side and the middle portion; the mounting base plate (4) is a rectangular flat plate with screw holes at four corners; and a left mounting bolt (5) and a right mounting bolt (6) are respectively installed at the upper left position and the lower right position of the base plate (4) and are used for installing the measurement apparatus to a transport platform.