Abstract: Base on the discussion of three kinds of basic technique of frequency synthesis such as direct frequency synthesis, phase-locked loop and direct digital synthesis, and several implementation technique of frequency synthesizer, a design of frequency synthesizer combining direct digital synthesis with a chain of frequency multiplier is given. This type of frequency synthesizer is especially suitable for ultra-high-speed frequency-hop system, and find a solution that the output frequency spectrum purity of frequency synthesizer is guaranteed on the condition of ultra high speed frequency hopping. Some experimental results are also presented in the end.