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In view of the existing field soil conductivity detection devices are mainly handheld, there are problems such as low detection efficiency and poor real-time performance. Based on the principle of current-voltage four-terminal method, a vehicle-mounted field soil conductivity on-line detection system was designed. The system mainly consisted of constant current signal source circuit, signal processing circuit, Arduino controller, GPS positioning module and vehicle-mounted sensor, which could detect field soil conductivity online. The performance of the system was verified by laboratory and field tests. The test results showed that the system had good stability, and the dynamic response time was about 540ms, and the maximum deviation of temperature drift caused by startup warm-up was 3.70%. The correlation coefficient R2 of the system detection accuracy was 0.7342 without considering the influence of temperature difference, and the detection accuracy R2 was 0.8645~0.9156 after eliminating the influence of temperature difference, which was higher than that of the commercial handheld conductivity detector with R2 of 0.6095. The influence of tractor vibration, sensor insertion depth, operation speed and soil solidity on the detection accuracy of the system was explored, the vibration state was relatively static, and the maximum error of detection data was 10.37%, and the error was mainly concentrated within the range of 0~10μS/cm. When the operating speed was no more than 5.0 km/h and the sensor insertion depth was greater than or equal to 10cm, the system could stably detect the field soil conductivity online, and the soil solidity of the test plot should not be too small to ensure that the sensor electrodewas in full contact with the soil. The system can provide technical support for the follow up research of real-time variable fertilization technology based on the online detection of soil conductivity.