The invention discloses a rail welding seam ultrasonic flaw detection device. The device comprises an ultrasonic flaw detector, a driving controller, a transmission mechanism, a first detection arm, a second detection arm, a third detection arm, a fourth detection arm and a draw bar, the first detection arm and the second detection arm are arranged at one side of the transmission mechanism and are driven by the transmission mechanism to slide in the horizontal direction, the third detection arm and the fourth detection arm are arranged at the other side of the transmission mechanism and are driven by the transmission mechanism to slide in the horizontal direction, the draw bar is horizontally arranged at the side face of the transmission mechanism, the transmission mechanism is connected to the driving controller and is driven by the driving controller, probes in the first detection arm, the second detection arm, the third detection arm and the fourth detection arm are connected to the driving controller, and the driving controller is connected to the ultrasonic flaw detector. The rail welding seam ultrasonic flaw detection device is compact in structural design and convenient to carry, has a more comprehensive detection range, reduces manual detection work strength, and effectively improves the rail welding seam detection efficiency and quality.