In this paper, the sideband suppression of a conventional soliton pulse is studied experimentally and theoretically based on a figure-9 passively mode-locked fiber laser. The sidebands of a conventional soliton could be sup-pressed by adjusting the polarization controller without reducing the output power of the soliton pulse. Three typical spectral forms with different degrees of sidebands suppression are experimentally obtained and the mechanism is analyzed theoretically. Based on the round-trip model, a theoretical model of a figure-9 passively mode-locked fiber laser is established. By changing the phase difference induced by polarization controller in the laser cavity, the experimental results could be confirmed. The above results have important reference value for improving the conventional soliton spectral output and eliminating the conventional soliton sidebands.