Based on the mode matching theory of electromagnetic field,the coupling efficiency of light from free space to Bragg fiber has been analyzed.It shows that the coupling efficiency is related to the core radius of Bragg fiber,the radius of focused facular and wave vector of the core.By optimizing the value of these parameters appropriately,the coupling efficiency can be improved.Simultaneously,the displacement between coupling lens and Bragg fiber is caused by the external vibration.The changed coupling efficiency caused by it has been analyzed.It plays an important role in analyzing the coupling efficiency of Bragg fiber,which can be applied to the fiber lidar system and optical communication system.