It is the inevitable trend of the development for the optical frequency comb based on the Ti: sapphire femtosecond laser to be replaced by that based on the Er: fiber femtosecond laser. The detection of the carrier-envelope offset f0 in the femtosecond laser is a key to the realization of an optical frequency comb. In this paper, the structure of the optical part of an Er: fiber optical frequency comb is introduced. The optical pulses from an Er: fiber femtosecond laser with a repetition of 230 MHz are first amplified with a two stage amplifiers, spectrally broadened in a highly nonlinear fiber, and then pass through a single-beam f—2f self-referencing interferometer. By optimizing the optical parameters, the f0 with a signal-to-noise ratio of 30 dB is obtained in the Er: fiber femtosecond laser, which is favorable for the realization of an Er: fiber frequency comb.