The invention relates to an ultrasonic phased array detection device and detection method of a vertical friction plate welding seam of a main cable saddle. The ultrasonic phased array detection devicecomprises an auxiliary detection robot and an ultrasonic phased array probe; a magnetic accessory is arranged on one side of the auxiliary detection robot, the auxiliary detection robot keeps close contact with a saddle seat through magnetic accessory, four magnetic accessories are arranged at the bottom of the auxiliary detection robot to keep contact with a vertical friction plate, two walkingwheels are arranged below the magnetic accessories at the bottom, and the auxiliary detection robot moves on the vertical friction plate through the walking wheels; and a probe support is fixed on oneend face of the auxiliary detection robot, and the ultrasonic phased array probe is fixed on the probe support. The ultrasonic phased array detection device provided by the invention effectively implements non-destructive testing of the vertical friction plate welding seam of the main cable saddle, the magnetic accessories can ensure the close contact with the saddle seat and the friction plate,the distance between the probe and the saddle seat can be continuously adjusted, and the coupling effect between the probe and the friction plate can be guaranteed.