The problem of large spectral linewidth of ultrashort pulse laser is studied. The pulse evolution process in the laser cavity is simulated by RP Fiber software. The effects of different saturable absorber versus the pulse width and spectral linewidth on the laser are studied, and the cavity length and fiber Bragg grating (FBG) parameters of the laser are optimized. Finally, a passively mode-locked picosecond pulse erbium-doped ring fiber laser based on WS2 saturable absorber is designed according to the optimization results. The spectral linewidth of the output pulse is compressed by the narrowband FBG, and a narrow linewidth ultrashort-pulse output with center wavelength of 1549.4 nm, pulse width of 171 ps and 3 dB spectral linewidth of 0.02 nm is obtained.