A wideband frequency synthesizer employing Phase-Refining technology and a method to calculate its phase noise performance accurately are presented. Based on linear transfer function and superposition principle, the analytic phase noise model of synthesizer is obtained. Based on the actual measured spectrum of the oscillator, the curve-fitting method is employed to predict the phase noise performances accurately. It is indicated that the main cascaded-offset Phase-Locked Loop(PLL) can reduce the in-band phase noise by minimizing the feedback divider ratio in the whole frequency range. The output frequency range of the experimental synthesizer is 2.1 GHz to 5.6 GHz with 1 Hz step size, and the measured phase noise are -114.7 dBc/Hz and -108.2 dBc/Hz at 10 kHz offset from carrier of 2.1 GHz and 5.6 GHz, respectively. The predicted and measured phase noise characteristics exhibit good agreement.