An experimental study has been performed to investigate the shear damage mechanism of aluminium alloy 2024T3 by an in-situ single shear test using scanning electron microscope (SEM). The observation of microstructural change and measurement of shear strain in mesoscale have been undertaken simultaneously. Hence, the shear damage mechanism and the shear stress–strain curve have been obtained. By measuring the deformed grid of a laser-marked sample, the shear strain distribution has also been found. On the basis of the experimental results, the relation between the shear damage evolution and the corresponding shear stress–strain state have also been obtained. It has been observed that two groups of microshear bands were formed in the shear process. When the shear stress is close to the shear strength of the alloy, slipping within the microshear bands occurs. The causes of macro-shear band formation and the interrelation and interaction among different types of microdefects are also discussed.