Abstract: Used as the adjunct device in treating heart failure,the flow field structure inside the ventricular assist device has an important role in the mechanism analysis and optimization design of the device.In the study,ventricular assist device was tested via a sac model and the flow field inside the sac was investigated by PIV (Particle Image Velocimetry).There were twen-ty-seven working conditions,which include three different sections of the sac model,with every section containing three operation frequencies as well as three suction gas volumes.The average velocity vector and turbulent shear stress were computed by processing the experimental data with overall average method.It is found that the flow field structure inside the device is mainly in the form of rotation and there are secondary flow near the inlet & outlet.Meanwhile,the loca-tion of the inlet & outlet has significant effect on the flow filed and the fluid around the inlet &outlet exchanges momentum strongly which leads to relative large Reynolds shear stress.The fluid can synchronously responded to the suction of different frequencies,and the center point of eddy moves continuously with time.