Liquid water is a kind of polar liquid, and has a strong absorption on the terahertz wave. The tiny change of soil moisture has a great influence on the transmitted terahertz wave. Accordingly, a method is proposed to measure the soil moisture based on time domain terahertz transmission spectrum. Soil terahertz parameters concept is proposed and the equations to calculate soil moisture are derived. Abundant results of soil samples are obtained with terahertz time domain spectroscopy, and the preliminary soil terahertz parameter value is given. By comparison with oven drying method, it demonstrates that this method is accurate and available.