A depth testing device for testing depth of a screw drive includes a supporting assembly which is for supporting the screw, a calibrating assembly, and a testing assembly. The calibrating assembly includes a calibrating element which presses on the top end of the screw. The testing assembly includes a testing device and a testing probe, the testing device includes a testing member and a testing pole stretching relative to the testing member. One end of the testing probe is received in the testing pole. The reading of the testing member is reset to zero first, then the other end of the testing probe is moved and is inserted into the drive of the screw, recording the reading of the testing member. The depth of the drive of the screw is calculated by the test reading subtracted by the thickness of the calibrating element.