Femtosecond laser is the most important part of an optical frequency comb, whose performance is mainly determined by the laser′s noise level, repetition rate, pulse width, spectrum and other parameters. A home-made erbium-doped-fiber femtosecond laser with ring structure and a repetition rate of 186 MHz is reported. This laser is designed for 9.2 GHz ultra-stable photonics microwave generation. The laser has an output power of 120 mW, and its spectrum center wavelength is in the range from 1550 nm to 1600 nm. The double-sideband relative intensity noise of the laser is -118 dBc/Hz at 1 Hz measured with a signal dynamic analyzer. From 10 Hz to 100 kHz, the relative intensity noise level is well below -130 dBc/Hz.