Abstract: The particle image velocimetry（PIV）is a non-intrusive technique to measure the flow field，which is widely used in the single-phase liquid flow as well as in the air flow.The basic principle of PIV is described，and experimental study on the application of PIV in measuring the flow field near the axial fan of the ventilation without wall is carried out.The experiment tested the flow field near the axial fan of the ventilation without wall when the inlet area of the model tunnel is 0.04 m2，0.03 m2，0.02 m2 and 0.01 m2.The result shows：it is reasonable and feasible to measure the flow field of the ventilation without wall by using PIV，which can get the flow field near the axial fan accurately；There is an obvious vortex in the air flow field above the outlet of the axial fan.The circulating air flow is generated between the outlet and the inlet of the axial fan with decreasing the inlet area of the model tunnel，and the air volume of circulating air and the inlet area of the model tunnel are negatively correlated.

Key words: PIV, Ventilation without wall, Measurement of the flow field, Vortex, Circulating air flow