Abstract: Flow control is an engineering technique that optimizes system performance by altering or directing fluid flow characteristics, and it has gradually become an important means of improving flow field quality. As one of the most promising active flow control technologies, dual synthetic jets (DSJ) efficiently entrain ambient fluids and are widely used in flow control. In this study, the instantaneous flow fields of DSJ with aspect ratios R_AR = 4.43, 10.00, and 15.63 at an operating frequency f_A = 650 Hz were measured using a high-frequency PIV system. The influence mechanism of the aspect ratio on the evolution of coherent structures in DSJ was analysed, and the characteristics of velocity profiles and entrainment mechanisms under different aspect ratios were investigated. The results indicate that increasing the aspect ratio reduces the velocity of DSJ, while enhancing the vorticity of the inner primary vortex evolution trajectory and weakening that of the outer primary vortex evolution trajectory in the minor-axis plane. Moreover, the vorticity of the primary vortices in the minor-axis plane gradually weakens with increasing aspect ratio, which exhibits the strongest vorticity but rapidly breaks down into small-scale vortices during evolution at R_AR = 4.43. In contrast, the primary vortices at R_AR = 10.00 and 15.63 in the minor-axis plane maintain their structures better, with significantly reduced intensity and number of small-scale vortices. Influenced by axis-switching behaviour, the vortex evolution trajectory in the major-axis plane first contracts and then expands. The spatial distribution of the periodic component of the time-averaged Reynolds shear stress in the major-axis plane shows positive/negative value regions opposite to those in the minor-axis plane, reflecting the three-dimensional effect of DSJ on the entrainment of ambient fluid. The velocity profile in the minor-axis plane exhibits a bimodal distribution. Increasing the aspect ratio accelerates the decay of the streamwise velocity. In the major-axis plane, the velocity profile exhibits peaks and troughs near the jet orifice, reflecting the asymmetry of blowing/suction, and revealing the formation mechanism of zero-net mass-flux jets.