Based on finite-volume direct averaging micromechanic s (FVDAM), which is a multi-scale finite element method for composite, a unifi e d macro- and micro-constitutive model for composite structures is developed. T he global stiffness matrix is constructed based on homogenization theory and co ntinuum medium mechanics, through which the meso-damage properties of fiber and matrix are transferred to macro-material. Hygrothermal stress fields are cal c ulated using the model. The results show that the efficiency of the FVDAM model is promoted for the reason of taking the surface-averaged fluctuating displac e ments as basic unknown micro-variables, and the total number of unknowns is d ecre a sed a lot. Because the equations are built on the basis of averaged meaning, th ere are discontinuities in the solved stress fields. The initial moisture conten t can reduce the thermal residual stress, but the hygroscopic stress would e xc eed thermal residual stress finally.