Abstract: Droplets have become an important research content of microfluidics. In order to realize precise regulation of the droplets' microenvironment, droplets were generated and trapped in long rectangular microcavities in a microchannel, and the internal flow field characteristics were experimentally measured by using a micro-particle image velocimetry (Micro-PIV) system. The effects of the Reynolds number (Re) on the droplet morphology, internal flow velocity vector fields and the distributions of shear stress inside the trapped droplet have been investigated. The results show that at Re=11.1, a vortex structure appears inside the droplet. When Re=33.3, the flow rate at the center of the droplet reaches a maximum value of about 10 μm/s. However, when Re=44.4, the vortex structure disappears and the average flow rate decreases. Meanwhile, the droplet size decreases as the Re increases. Moreover, Re has no significant effect on the internal shear stress of the droplet, and the average value of the shear stress is extremely low (< 1.5×10^-4 Pa).