In order to reveal the characteristics of droplet mixing in a microfludic T-junction, the dynamic mixing process in droplet was studied by numerical simulations and experiments. The microfluidic chip was manufactured by soft lithography based on polydimethylsiloxane (PDMS), and the experiment phenomena of droplet formation and mixing process were analyzed with microscope system. The research results showed that the mixing efficiency was significantly influenced by the twirling effect which controlled the initial distributions of mixture during the droplet formation stage. While the droplet was separated from the dispersed phase in the junction region, the combined effect of viscous shear exerted by the continuous phase and the re-orientation of flow induced a vortex in the dispersed phase. As the two-phase interface in the droplet was rotated by the twirling effect, the mixed solutions were distributed to the front and back parts of the droplets. Therefore, the internal recirculation flow can bring in convection mechanism thus improving mixing. The twirling effect was noticeably influenced by the velocity of continuous phase. When the velocity of the continuous phase was slow, the twirling effect cannot distribute effectively for long droplets because of the large amount of fluid contained in the droplets. In contrast, the twirling effect appeared to be very effective for short droplets as the amount of fluid was small. When the flow velocity of the continuous phase was 0.04μL/min, the mixing index was 0.82, which was two times higher than that of the mixing index when the flow velocity of the continuous phase was 0.005μL/min. This study provided the insight of droplet formation and mixing performance inside the droplet in droplet-based microfluidics and it should be helpful for microdevice design and optimization.