The phase noise level of the frequency source directly restricts the upper limit of radar performance, thus low phase noise frequency synthesis is a key technology for high performance radar systems. Existing low phase noise frequency synthesis methods are often realized by high order frequency doubling, and the overall performance is heavily dependent on the low phase noise Oven Controlled Crystal Oscillator(OCXO), therefore the cost is high. In this paper, a low additional phase noise frequency synthesis method is proposed, the technique of minimizing the additional phase noise on the link is adopted, and an ordinary OCXO is employed to cascade the low phase-noise amplifier, comb generator, and phase-locked loop to ultimately realize the frequency synthesis with low phase noise. The measured data show that the proposed method uses a 100 MHz OCXO as a reference, the time jitter in the integration interval [1 kHz,30 MHz] is 11 fs, the phase noise of the frequency synthesized at the 5.8 GHz carrier is -119 dBc/Hz@1 kHz, the time jitter in the integration interval [1 kHz,30 MHz] is 13.7 fs, the total additional time jitter is 8.17 fs, and the additional phase noise is only 1.9 dB. It reaches the industry-leading level, which can effectively improve the imaging performance of millimeter-wave radar systems and outperforms the traditional frequency synthesis methods.