Abstract: The design,set-up and test of an electrostatic deflector used in the accelera-tor mass spectrometer(AMS)are described.The main characteristics of electro-static deflector are as follows:The deflection angle is 15°.The radius of curva-ture and length is 360 cm and 100 cm,respectively.The gap between the electrodesis 3 cm.The positive and negative voltage on the two electrodes reach 92.4 kVand 80 kV.respectively,and the electric field is 57.5 kV/cm.