The invention discloses a middle-supporting type plastic cylinder three-point positioning mechanism which comprises a frame. Upper supporting plates are fixed to the top faces of the two sides of a top plate of the frame, and a connection supporting plate is fixed to the front portion of the top plate of the frame. A guiding connection sleeve is inserted into a central through hole formed in the middle of the connection supporting plate, the outer side wall of the rear end of the guiding connection sleeve is welded and fixed to the inner side wall of the central through hole, and a middle-supporting cylinder is fixed to the front end of the guiding connection sleeve. A middle-supporting column is fixed to a push rod of the middle-supporting cylinder and inserted into the guiding connection sleeve. A cylinder is clamped automatically in a three-point positioning mode through two arc grooves and an upper arc groove by means of clamping air cylinders and a lifting air cylinder, and the middle-supporting column is inserted into the cylinder to achieve middle-supporting; it is guaranteed that the cylinder cannot deform in cutting, the cylinder is fixedly and firmly clamped, and the effect is good; meanwhile, clamping blocks are convenient to connect, mount, dismount and replace.