A method to broaden the spectrum by direct phase modulation is proposed. The effects on the spectrum of modulation depth,modulation signal width, synchronization precision and different signal forms are investigated by numerical simulations. The results show that deeper modulation and more narrow modulation signal width lead to a more obvious effect and a broader spectrum of single modulation; different modulation signal widths require different synchronization precisions, and the narrow signals need a high-precision synchronization; compared to the parabolic signal, sinusoidal signal can lead to an obvious frequency modulation to amplitude modulation, which can bring down the compressibility and the signal to noise ratio. The experiment results of the central wavelength match well the simulation results.