A line testing device applicable to a distributing frame comprises a casing, wherein pins are arranged at the front end of the casing; an internal connection circuit is arranged in the casing, is electrically connected with a measuring device through a wire and comprises an input terminal a and an input terminal b which are connected with the pins, the input terminal a is connected with an output terminal A through a thermistor R1 and a switch S1 which are sequentially connected in series, the output terminal A is grounded through a first discharge tube, the input terminal b is connected with an output terminal B through a thermistor R2 and a switch S2 which are sequentially connected in series, the output terminal B is grounded through a second discharge tube, a node between the first discharge tube and the second discharge tube is connected with an anode of a light-emitting diode D1 through a switch S3, and a cathode of the light-emitting diode D1 is connected with a terminal S. The device can solve multiple problems caused by existing line adjusting tests and can effectively test the fault distance.