In order to improving the sensitivity and selection of modern communication system, put forward a predication methods for PLL’s phase noise. The phase noise performance of current charge pump PLL frequency synthesizer is analyzed, on this base, the traditional and NSC’s phase noise predication method are proved. Also, the two methods are modified, and the application region is specified in this paper. At last, two low phase noise PLL design is provided to prove the phase noise predication methods is relatively accurate. It can also short the design time and reduce the cost, and provide some meaningful instruction in low phase noise PLL designs.