In order to improve the calculation accuracy of moisture index of subgrade in highway climatic zone Ⅱ₁, an optimal calculation method of subgrade moisture index was established by employing the Food and Agriculture Organization Penman-Monteith (FAO-PM) method instead of the Thornthwaite method to estimate the average monthly potential evapotranspiration (PE) of soil subgrade, which considers the evapotranspiration of soil subgrade as zero under negative temperature conditions. After analyzing the influence of meteorological parameters on the monthly PE of soil subgrade, the applicability of the proposed method was verified and the calculation process of subgrade moisture balance was simplified. Based on the seasonal change characteristics of subgrade moisture, a staged moisture index calculation method was proposed to refine the range of subgrade moisture index in climate zone Ⅱ₁. Results show that when the temperature was below zero, the average monthly PE of the subgrade was positively correlated with average sunshine hours, average temperature, and average wind speed, but negatively correlated with average relative humidity. Moreover, the total PE accounted for 9.8%-15.7% of the total evapotranspiration throughout the year. The subgrade moisture equilibrium states were classified into three types: runoff, no runoff with or without drought, for which the most unfavorable seasons could be distinguished by the four-stage statistical analysis of the moisture index. For the seven meteorological stations studied, the minimum and maximum moisture indexes of different soil groups were calculated as -16.5 and 33.2 respectively, which are less than the current standard recommendation (-8.1-35.1), indicating that the subgrade moisture in these regions are more arid than expected and gradually decreases from southwest to northeast. The method can not only be used to effectively calculate the evapotranspiration of soil under negative temperature, but also ensure the accuracy of subgrade moisture index estimation in climatic zone Ⅱ₁.