Low-noise microwave signals are of vital importance in fields such as cold atomic optical clocks, photon radars, and remote synchronization at large facilities. Here, we report a compact all-optical-fiber method to the generation of low noise microwave signal, which uses fiber loop optical-microwave phase detector to coherently transfer the frequency stability of the ultra-stable laser to the microwave. Combining a narrow linewidth optical frequency comb and a fiber loop optical-microwave phase discriminator, a tight phase-lock between 7GHz dielectric oscillator and optical frequency comb is achieved, the remaining phase noise of the synchronized optical pulse sequences and the microwave signal is -100dBc/Hz@1Hz, and the timing jitter is 8.6fs [1Hz-1.5MHz]; by building two sets of low-noise microwave generation systems, the measured residual phase noise of the 7GHz microwave is -90dBc/Hz@1Hz, and the corresponding frequency stability is 4.8×10-15@1s. These results provide a novel idea for the generation of low-noise microwaves based on optical coherent frequency division.