Soil conductivity is an important parameter of soil environment and can be used as an important data to evaluate soil fertility and productivity and make precise fertilization prescriptions. Based on the principle of “four-terminal method”, a vehicle-mounted fast detection and acquisition system for soil conductivity in field was designed. The system included AC constant current signal source, signal detection and conditioning circuit, and GNSS positioning system, which can realize fast detection of soil conductivity in different areas. The influence laws of soil water content, soluble salt conductivity, electrode penetration depth and soil temperature on four-terminal electrode output signal were explored through four-factor and five-level center combination experiment, and the predictive regression equation of conductivity of soil solution (ECw) was established. The regression coefficient R2 of the equation was 0.9961. The field tests were carried out with the system installed on the soil conductivity testing device. The system detection data were calculated based on the regression equation and compared with the actual values of the laboratory soil sample testing. The test results showed that the regression model established in the laboratory could be used to calculate the conductivity of field soil solution. The sensor data obtained by the system on the same or similar path were stable. The predicted value of soil solution conductivity had a similar trend with the actual value in this region, and could be used for fast and real-time detection of soil solution conductivity (ECw) in field. The research result can lay a foundation for further research on variable fertilization control technology based on soil conductivity.