A soil electrical conductivity online real-time detection system was proposed. A full set of design scheme of AC method to measure conductivity was introduced, which included the design of a signal source, a stable AC current power supply circuit, probe, sensor signal conditioning circuit and acquisition system. The AC four-electrode method has a better detection performance compared with DC four-electrode method. The test system performance reached the same level compared with products from abroad and it had a more cost efficiency compared with the similar foreign products. Compared with Hydra Probe II sensor, the absolute error was -22.62~29.41 μS/cm, relative error was -2.2%~2.4%, the maximum relative error was 2.4%, the accuracy was the same. Compared with mercury thermometer, the temperature sensor had the absolute error of -0.12~0.23℃, maximum relative error of 2.3%，which can satisfy the accuracy of measurement. The soil in Beijing area was selected as test samples, the system’s measuring range was 0~1 520 μS/cm, monitoring error was -2.2%~2.4%, the dynamic response time was 544 ms, the measurement bandwidth was 1 MHz, the performance can meet the measurement requirements, which can provide a high precision measuring tool for precise agroforestry.