The utility model discloses a small rail car mileage detects rotary encoder's drive mechanism, the diaxon frame plate that corresponds including rotary encoder and shaft coupling, position, the synchronous pulley that diaxon frame plate bottom both ends axle supported and close ring tooth just as ambulacrum, middle synchronous pulley and axle bed board rather than engaged with, its shaft of synchronous pulley at diaxon frame plate bottom both ends is fixed with the same drive pulley respectively, meshes and meshes mutually with middle synchronous pulley in the ring tooth shape transmission synchronous belt that closes of two drive pulleys. Rotary encoder and two synchronous pulley that middle synchronous pulley transmission is connected set up two separately and close ring tooth just as the ambulacrum, consequently can avoid anti -skid close ring tooth just as the ambulacrum take place not hard up, warp the interference of production to rotary encoder. The utility model has the advantages of simple structure, be favorable to improving mileage measurement and repeated positioning accuracy's salient.