With the rapid development of detection technology, a variety of pH value detection methods was invented. However, there were many pH value detection techniques, but few of them can be used in field. Thus a soil pH value online realtime detection system in this situation was designed. It was found that the measurement of pH value sensor was greatly influenced by soil moisture and temperature. In order to reduce the influence of soil moisture and temperature on soil pH value measurement, a model of moisture and temperature compensation for pH value sensor was proposed. Soil pH value measurement values were linearly analyzed by the least square method with soil moisture and temperature. The results showed that the absolute error of pH value measurement caused by temperature and soil moisture was reduced by more than 84.5%, and the variation range of pH value measurement was not more than ±0.1. Compared with ZD-18 soil acidity meter, HYSWR-ARC-12V soil moisture sensor and mercury thermometer, three determinant coefficients were all above 0.99. It proved that the performance of the system was equivalent to mature products. In order to ensure the applicability of soil pH value measurement in natural environment, it was explored that the threshold of soil moisture for pH value sensor was effectively measured, which proved that the pH value sensor can be effectively used when the soil volume moisture was more than 5%. Experiments showed that the pH value sensor can be effectively measured in the range of 3.06~10.36, and the detection error was -1.53%~3.51%. The design of a highaccuracy soil pH value online realtime detection system was realized.