This standard specifies a detailed method for determining the moisture absorption performance of polymer-based composites and the adjustment of equilibrate state. It covers the test device, sample preparation, test conditions, procedures, calculation of results and reporting requirements. It is suitable for the adjustment of moisture absorption equilibrate state before the test of mechanical properties, and the determination of equilibrate Moisture absorption and wet diffusion coefficient. The test process determines the moisture absorption equilibrate state by placing the sample in a wet environment and recording the change of Moisture absorption over time. This standard provides requirements for test devices such as electric air blowing dryers to ensure the accuracy and repeatability of the test process.