As an expansion existing in an open channel, a recirculation zone is formed downstream the expansion, and then a mixing layer is formed between the main flow zone, where vortex structures exist which dominate the material and momentum exchange process between the former two zones. This research focuses on the vortex dynamics downstream two-side lateral expansions in shallow open channels. First, the length of the recirculation zone is measured in a sudden lateral expansion channel by particle image velocimetry and also simulated by large eddy simulation. Second, to analyze the vortex dynamics, the flow fields are reconstructed based on proper orthogonal decomposition (POD). POD results indicate that the vortex energy decreases as S increases. Furthermore, the Reynolds stress and turbulent kinetic energy characteristics are compared among different S conditions that could be linked to the vortex energy dissipation process. Finally, a one-dimensional spectrum analysis is performed in the center of the mixing layer that reflects the vortex shedding frequency of the vortex. In summary, in the small S condition, the flow is significantly more confined by the free surface, the vortex energy peak frequency decays more slowly, and the vortex energy dissi-pation slows down.