Abstract: The development of electronic system brings more demands to the accuracy of frequency source, frequency resolution, conversion time and frequency spectrum purity. Phase-locked frequency synthesis technology has small volume, simple circuit and high stray inhibition degree. It has narrow-band filtering capability, so it is used widely. But there is a disadvantage between the frequency step and conversion time constraints. DDS has a very high frequency resolution, extremely fast frequency converting. But its upper limit of output frequency is not high. The two methods have their own advantages and disadvantages. So use DDS + PLL technology solutions which can both retain the advantages of the phase lock loop, and make up for the inadequacy of the phase locked loop. Based on the idea of mixed frequency synthesis method is this paper proposes the improvements.