Abstract: Since it is hard for the available gauge of AC 4 Pole Method to measure the soil of high resistivity, two methods of expanding the measuring range are set forth in the paper, namely, Pole-Distance (PD) regulating and Parallel Resistance (PR) regulating. PD regulating is a method of adopting different pole destances in order to change the measuring range of resistivity instrument. PR is to parallel a known value resistor between current and pole or between potential and pole so as to help ZC-8 instrument to enlarge its measuring range of high resistivity. Formulas for both methods have been worked out with the discussion of PR's r value definition and the influence on measuring the calculated value as well as the principle of selecting R₀ and its practical application. The improved AC 4 Pole Method can help solve the problem that the conventional one couldn't, which was improved during the investigation of soil corrosion in Tarim Oil Field, Xinjiang.