A method of measuring moisture content for plant material powder based on HM1520LF capacitive humidity sensor was introduced. The raw data sequence of moisture content was acquired by measurement system which consists of capacitive humidity sensor and data acquisition card. After excluding alternative singular by one rank difference method, inertia filtering method was employed to process the data sequence. By accordingly comparing 13 measured values of moisture content with corresponding true values from the baking oven, the maximal relative error of moisture content between measured values and true values was 1.61%, and the average relative error was 0.76%, both less than 5% (confidence interval 95% for 5 samples). As a result, the method can be used to determine the low moisture content of plant material powder. 