This study proposes a structural design for a breath soliton erbium (Er)-doped fiber laser under a relatively low (100 MHz) repetition rate to obtain a pulse output having narrow pulse width. An output pulse with a large breathing ratio can be ensured by shortening the lengths of the positive and negative dispersion fibers with a large dispersion coefficient in the cavity. And, a fiber with zero dispersion is introduced to reduce the repetition rate of the laser. The energy loss of the pulse in the cavity is reduced using a fiber optic mixer, which integrates the wavelength division multiplexer and the isolator with the forward pumping method. The distribution of the negative dispersion fiber in the cavity is optimized and designed to determine the output position of the laser. Based on this, a breath soliton Er-doped fiber laser having a repetition rate of 100 MHz is constructed; the laser's output spectrum and direct output pulse width are 112 nm and 68 fs, respectively. After dispersion compensation, the laser's output pulse width is 39 fs, and its average power of output pulse is 94.5 mW at a pump power of 900 mW.