The invention relates to a ground device used for a relief well electromagnetic detection and positioning tool. The ground device comprises a cable which is arranged on a Martin-Decker photoelectric coder in a winding way. One end of the cable is connected with an acquisition device. The ground device also comprises a test data processing subunit, a depth data acquisition subunit, a data communication subunit and a control system. The other end of the cable is connected with the test data processing subunit. The test data processing subunit receives test data transmitted by the acquisition device arranged in a relief well and transmits the test data to the control system through the data communication subunit. The depth data acquisition subunit receives pulse data transmitted by the Martin-Decker photoelectric coder and converts the pulse data into depth data of the acquisition device in the relief well and then transmits the depth data to the control system through the data communication subunit. The control system comprises a data initialization subunit and a data display subunit. The data initialization subunit receives the test data and the depth data transmitted by the data communication subunit and respectively transmits the test data and the depth data to the data display subunit to be displayed.