Abstract: Servo circuit is the core control component of the atomic frequency standard. In this paper，an improved servo system method is described in detail of DDS achieves the wide range synthesizer control，and synchronous phase based on the traditional technology. Through the mentioned improvement，on one hand，overcomes the lock losing problem in traditional technology，on the other hand，makes the atomic frequency standard have the function of function mode indication and judgement.

Key words: atomic frequency standard, servo, locked indication