A multi-parameter compounded testing system was proposed, which could collect soil moisture, electrical conductivity and temperature at the same time. An intensive research was carried out on soil moisture using standing wave ratio method, on electrical conductivity using four-electrode method, and on temperature using platinum resistor method. The soil in Beijing area was selected as sample; the correlation coefficients between measured value and value output by sensor for soil moisture, temperature and electrical conductivity were 0.998 3, 0.999 8 and 0.999 1, respectively. The dynamic response times and standard deviation of stability for the above three parameters were 460 ms, 13 s, 2.28 s and 0.011 5, 0.001 4, 0.010 3, respectively. Both stability and dynamic performance can meet the measurement requirements. The anti-interference performance of the proposed system was analyzed. With time-sharing power supply, the systems interference mainly existed between soil moisture and temperature. The soil temperature probe, which was considered as an abnormal medium, will cause increasing of soil moisture. This error can be compensated in data processing. The generally recognized soil moisture sensor, soil temperature sensor and soil electrical conductivity sensor were selected to compare with the proposed multi-parameter compounded testing system. The results demonstrated that the system can meet the requirements of real-time online measurement, and provide a portable tool with high accuracy for measurement of soil parameters.