Evaporation of a sessile droplet containing nanoparticles plays a crucial role in engineering. However, the internal flow of an evaporative droplet may be influenced by various factors. Therefore, it is necessary to explore the mechanisms of fluid flow, especially the evolution of thin liquid film near the triple line of an evaporating droplet. This paper describes an experimental study of fluid flow and thin film evolution near the triple line of a sessile droplet when it was mixed with another droplet of different size. The temporal and spatial evolution of thickness in the thin film near the triple line is obtained by using the sub-region method developed from the total internal reflection fluorescence microscopy. The experimental results show that the spatial variation of the local film thickness can be linear or oscillating depending on the mixing position of the droplets. When the mixing position is at the droplet apex, the film thickness near the triple line fluctuates drastically in an oscillating mode, indicating that the mixing of the small droplet causes a strong disturbance in the thin film region. By using the velocimetry technique, the distribution of near wall velocity in the sessile droplet during mixing process is obtained, which provides the basis for velocimetry near the triple line. This work helps to gain insight of the thin film evolution and the velocity field near the triple line on the mixing processes of droplets.