A strap tightener has an auto-pulling device connected to a seat. The auto-pulling device includes a housing and a reversing device. The reversing device is rotatably received in the housing. The reversing device has a reel received in the housing to connect with one end of a strap. A spiral power spring is connected between the housing and the reel. Consequently, the strap can be automatically wound around the reel after the strap tightener is used. To collect the strap is convenient and the operating time of collecting the strap is saved.