Using smoke wires and PIV technique,the near wake(X/D<5 )of circular disk with an aspect of D/H =5 at Re=22400 was experimentally studied.Flow visualization using smoke wires showed that there was a stable recirculation zone and stochastic,distorted and tilted three-dimensional vortices behind the disk.By statistic average analysis of a series of cartogra-phies obtained by PIV technique,it was found that the mean flow field was symmetric with a re-circulation zone in length of 2.1D.POD analysis was applied to the PIV data,reconstruction of the first pair of eigenmodes manifested that the peaks of u2 and uv located in the shear layer be-tween the free stream and the recirculation zone,and the peak of V2 appeared near the stagnation point of the recirculation zone,where vortices were shedding displayed by smoke wires.As a conclusion,vortices generation and shedding in the wake originated from shear layer instability and its interaction with the recirculation zone.The first and second POD modes represent only 10.9% and 10.2% of total turbulent kinetic energy,which means that turbulence structures is relatively stochastic.The first ten modes represent 45.9% of total energy,therefore,the mean Reynolds shear stress can be well described.