A method of controlling spectrum based on all-fiber multi-pass phase modulation is demonstrated experimentally and theoretically. Numerical simulation results indicate that modulation times, modulation depth, shape and width of the modulation signal and synchronized precision between optical pulse and modulation signal have influences on the central wavelength, bandwidth as well as the shape of the output spectrum. The input spectrum with bandwidth of 0.03 nm is broadened into 2.238 nm after multi-pass phase modulation, and the experimental results match well with the simulation results. The corresponding relationship between the modulation signal waveforms with different phases and the output spectra is obtained, and the feasibility of the spectral characteristics control is verified.