The invention provides a portable ultrasonic detector which comprises a host (1) and a display screen (2). A vent port (11) is formed in the left/right side wall (12) of the host (1). The display screen (2) is connected to one end of the host (1) through a pivotal shaft (6). A dustproof assembly is further included and comprises a shielding piece (3) for shielding the vent port (11). A control system for controlling the shielding piece (3) to be opened or closed is further included. When the display screen (2) is turned off, the control system controls the shielding piece (3) to shield the vent port (11). When the display screen (2) is turned on, the control system controls the shielding piece (3) to open the vent port (11). The portable ultrasonic detector can automatically open or shield the vent port according to turn-on or turn-off of the display screen, and it is avoided that when the portable ultrasonic detector is used in field, dust enters the vent port, and consequently the performance of the detector is reduced.