The present invention discloses a ship draught detection apparatus and its detection method. The ship draught detection apparatus includes two servo motors, a guide rail, a sonar detection device, a traction device, a locating device, and a control device. Compared with the provision of a series of sonar detection devices in the prior art, the provision of one sonar detection device reduces the cost and there is no mutual interference between adjacent sonars. Meanwhile, because a lock gate width of a ship lock is greater than a width of the ship, in order to improve the detection precision, the locating device is configured to predict in advance an accurate position of the ship when passing the ship lock according to the position of the ship and the orientation of a ship bow, and the control device controls the rotation speed and the direction of the two servo motors, and makes the sonar detection device move along the guide rail through the ropes, so that the sonar detection device is located just below the ship when the ship passes the ship lock, thereby improving the detection precision.