Abstract: Voids are comment defects generated during the manufacturing process and highly sensitive to moisture in the hygrothermal environment. The presence of voids will change the stress field and moisture field, which has deleterious effects on the mechanical performances. The mechanical properties of carbon fiber reinforced polyamide 6 (CF/PA6) composites after hygrothermal aging under different temperature immersion environment were tested to study the effects of temperature and water absorbed content on the mechanical properties. The correlation function between water content and mechanical parameters was established. Based on the random sequential adsorption algorithm (RSA), the representative volume element (RVE) model with random distribution of fibers, interfaces and voids was established. The quantitative effects of voids content on strength and modulus under the loading of transverse tension, compression and shear were investigated, by introducing a degradation factor dependent on water content into the constitutive model, and the failure mechanisms before and after hygrothermal aging were revealed. Conclusively, before hygrothermal aging, voids induce the decrease of mechanical properties due to stress concentration, and every 1% increase in the voids content results in a 6.4% decrease of transverse tensile strength. However, matrix degradation due to the absorbed water content after hygrothermal aging is the dominant factor, and the corresponding rate is 3.86%.