The determination of residual water content is vital for the quality control of high explosives, such as octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX). However, conventional analytical methods are time-consuming and inaccurate. We have proposed a simple, sensitive, and accurate method for moisture determination based on terahertz (THz) time-domain spectroscopy. A model based on effective medium theory was implemented by a third-order extension of the Landau, Lifshitz, and Looyenga model to predict the moisture content of HMX based on THz spectroscopy. The predicted values of moisture content based on the model were found to be in excellent agreement with the results obtained using gravimetric measurements. Overall, the results proved the effectiveness of the proposed method for rapid and accurate moisture determination of HMX.