The invention provides a back massager which comprises a cover body, a movable seat, massage heads, driving devices, reversing components and hollow guide pieces; the movable seat is arranged on the cover body in a sliding manner; the driving devices are arranged in the movable seat and drive the massage heads to move; the guide pieces are mounted on the cover body and are provided with ring-shaped guide parts and inner gear racks; each reversing component comprises a driving gear, a driven gear, a rotating piece and a rotating shaft; the rotating shafts are pivotally connected to the movable seat and are provided with extending ends extending into the inner gear racks; the rotating pieces and the driving gears are mounted at the extending ends; the rotating pieces are provided with sliding parts extending to the guide parts and sliding along the guide parts; and the driven gears are pivotally connected to the rotating pieces, and the two ends of the driven gears are in meshed transmission with the driving gears and the inner gear racks respectively. According to the back massager provided by the invention, the driving devices drive the driven gears to move along the inner gear racks unidirectionally through the rotating shafts and the driving gears, the moving driven gears drive the movable seat to slide in a line to and fro through the rotating pieces, and correspondingly, the normal service life of the back massager is prolonged.