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Focal spot size is an important factor that affects the spatial resolution and the projection image unsharpness.More seriously,the size and position of the focal spot may change as time goes or when the tube voltage or power set change especially in micro cone-beam CT.It is necessary to monitor the focal spot state in real time when doing CT scanning.In this paper,a tungsten wire with the diameter of 100 microns is used to test the state changing.The wire is placed between the tube and the fiat panel detector.After the tube is opening,the projection of the wire is acquiring in time.Based on the wire projection edge or midline position movement,we can acquire the information of the focal spot poison changing direction.In addition,based on the wire width change,the information of the focal spot size can be obtained.A serious of different voltages and powers are set in the experiment.When the tube power changes from 3 watt to 15watt at fixed tube voltage,the wire projection position and width change a little.When the tube voltage changes from 100kV to 200kV at fixed power,both of the wire projection edge and the width have varied.The real-time observation of the wire projection status can be used for the analysis of the spatial resolution of the micro cone-beam CT system.