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Through-thickness microstructure and mechanical property of AA 7055-T7751 aluminum alloy plate were investigated by using electron backscattered diffraction (EBSD), transmission electron microscope (TEM) and small angle X-ray scattering(SAXS). The results indicate an inhomogeneous distribution of microstructure through the thickness. The degree of recrystallization decreases gradually from 69% to 19.1%, as deepening from the surface to the center of the plate. The size of subgrains decreases from 10 μm at the surface to around 2 μm at the center. Strong texture of rolling type is observed near the center but the intensity decreases gradually as nearing the surface and the shear texture becomes the dominant. High density of plate-like η' phases are observed in the alloy, indicating the sufficient precipitation. η' precipitates of this condition are around 3.7 nm in radius, 1-3 nm in thickness and are found coherent with the Al matrix with a coherent strain of 0.0133, showing a strong strengthening effect. The heterogeneity in grain scale does not influence the distribution and the morphology of precipitates. The yield strength (L direction) varies linearly along the thickness direction of the plate, fitting an equation of <i>σ</i>_y=-38.7<i>S</i>+604.8 (0≤<i>S</i>≤1). The variation of yield strength is related to the heterogeneity of grain structure.