PIV measurements are carried out on the thermally stratified flows under different horizontal shear intensities in a confined space. Proper orthogonal decomposition (POD) is used to analyze the structures of these thermally stratified flows. Results show that, at the conditions without forced horizontal airflow, the low-order POD mode constitutes a high proportion of the total energy of the flow, and the horizontal flow is the dominant flow structure. The horizontal shear drives the energy to dissipate from large structure to smaller structures. Relative to the conditions without forced horizontal airflow, the ratios of vertical kinetic energy to horizontal kinetic energy become higher for each order of POD mode in the conditions with forced horizontal airflow. This indicates that horizontal shear can result in stratification instability and enhance the mass exchange in the vertical direction.