The air jet entrainment of stratified air conditioning system in large space building can cause the transfer of heat from the upper part of conditioned area to the lower part of non air conditioning area, which is one of the reasons leading to the increased load of air conditioning system. The article built the platform of saline model experiment of circular jet movement,and used the particle velocity measurement system (PIV) to extract information of nozzle flow field near the shear layer entrainment velocity vector field.With the help of proper orthogonal decomposition method (POD), it analyzed the coherent structures and energy distribution, vertical and horizontal velocity characteristic value proportion in the shear layer. The system can provide experimental and theoretical reference for the later research of transfer heat in large space caused by jet entrainment.