Abstract: Cesium beam tube is an electrical vacuum device used as atomic frequency discriminator,which requires an external circuit to provide a frequency signal of 9 192.631 770 MHz to excite the cesium atom transition.In this paper,a precision frequency synthesizer is designed,which makes use of the characteristics of the DDS principle of nonlinear out-put.It used the AD9858 with filter circuit structure,to achieve the output frequency of 87.368 23 MHz while the 80 MHz frequency signal as a reference signal.The intermediate frequency signal is used for the phase-locked loop to generate the mantissa frequency that excites the cesium atom transition frequency.The circuit has the advantages of simple structure and small volume,and the control parameters are easy to calculate while ensuring the output signal quality.At the same time,it can adapt to the space environment requirements of the space-borne cesium atomic clock,and it has good temperature performance in vacuum environment.The design has been successfully applied to the spaceborne cesium clock.After long-term on orbit test,it can be seen that the accuracy of the spaceborne cesium clock is-1.52×10^-12 with the frequency design of the high-precision frequency synthesizer proposed in this paper.