The invention provides an automatic detector for use in formation of power transmission line foundation pits. The automatic detector comprises an over-pit control unit, an intra-pit measuring unit and a computer. The computer is connected with the over-pit control unit. The over-pit control unit is fixed in the center of an opening of a foundation pit through a support. The intra-pit measuring unit is disposed on a center line in the center of the foundation pit and connected with the over-pit control unit through a steel wire. The over-pit control unit drives the intra-pit measuring unit to move up and down through a direct-current reduction motor under control of the computer. The automatic detector is small in size and light in weight, is applicable to quick accurate automatic measurement of the constructional size of the foundation pit, can protect personal safety and is conveniently applied to the measurement of foundation pit size under complex geographical conditions.