This standard is part of a series of standards for testing reinforcement products, specifying the method for determining the moisture content of glass fiber, carbon fiber, and aramid fiber products. The test employs the drying and weighing method, with its core principle being the use of an analytical balance (requiring a division value of no greater than 0.1 mg) or an instrument of equivalent precision to weigh the specimen before and after drying, and then calculating its moisture content. This method is applicable to various forms of reinforcement products, such as continuous filament yarns, rovings, chopped strands, mats, and fabrics. Its application is of significant importance for evaluating material performance and controlling product quality.