Aiming at solving the problems existing in the traditional water quality detection process with the limited measurement range and the instability of the measurement system, a wide-range and high-precision electromagnetic conductivity measuring device was designed, and the range switching control method was optimized to achieve wide-range, high-precision and high-stability detection of solution concentration. The large-range conductivity measurement system is rather sensitive to the threshold setting during range switching. Firstly, through experiments, the optimal threshold inflection point of the solution conductivity is optimized and selected. For the signals that frequently switch ranges near the threshold, the method of locking the range by comparing point by point is adopted to lock them in a certain range interval, so as to improve the stability and reliability of the system. Secondly, considering the low accuracy of signals in the locked area, the data fusion algorithm based on the fuzzy membership function is used. After conducting the least root mean square error experiments on ten kinds of conductive solutions, the optimal fuzzy interval is selected to complete the fusion processing of the locked signals. Finally, multiple sets of measured data were substituted into the proposed method to verify its effectiveness. The experimental results show that the fusion algorithm after parameter optimization can achieve accurate measurement of the conductivity at the threshold edge, with the maximum measurement error being 0.85%, which is significantly better than the traditional single-range measurement method with a maximum relative error of 2.86%. In addition, the detection range of the designed conductivity sensor is from 0.1 μS/cm to 2000 mS/cm, and the relative errors in the full-range tests are all less than 1%. It indicates that the method proposed in this paper can achieve high-precision and wide-range detection of solution conductivity.