In a method for detecting carpal tunnel using an ultrasonic detection device, the palm is placed on a flat surface, and the fingers are naturally stretched out to form a “5” shape; a mark is placed 0.5 cm above the crease of the palm (Distal wrist crease), and the probe unit of an ultrasonic detection device is placed at the short axis position of the wrist joint. By rotating the probe unit, the probe unit, the marker, and the index finger are on the same axis (index finger axis), so that the image of the carpal tunnel section of the palm can be obtained on a display of the ultrasonic detection device. Accordingly, the detection method is accurate and efficient, correctly guides students and doctors to find the position of the carpal tunnel correctly, and avoids the purposeless search for the position of the carpal tunnel by the probe on the palm, and subsequent treatment.