A significant number of investigations have demonstrated that clay subgrades are subjected to a combination of traffic loads and environmental changes following their implementation. The moisture content of the subgrade has undergone large fluctuations. It is far beyond the range of the optimum moisture content. Consequently, a profound comprehension of the regularity of the moisture content of clay subgrade is of paramount importance. In this study, moisture content data were collected by means of a field sensor array and subsequently corrected through the use of calibration tests. Finally, the evolution law of moisture content in subgrade soil with different filling heights was investigated. The results show that the accuracy of the moisture content data can be effectively improved by calibrating the sensor, with a maximum error of 3.2%. The change of moisture content along the depth direction of the subgrade of high fill shows a trend of increasing then decreasing and then increasing, while the subgrade of low fill shows a trend of increasing all the time. The moisture content of the high fill subgrade at a depth of 0.5 m from the top surface of the subgrade decreased by approximately 3% following two freeze-thaw cycles. The moisture content at depths of 2 m, 3.5 m and 5 m from the road surface increased by approximately 1%, 2.4% and 2.5%, respectively, following two freeze-thaw cycles. This study provides an important reference for the design and construction and quality control of clay subgrades in the seasonal freezing region.