Laser is a kind of common light source in modern optics. It is of great significance to analyze its spectrum width characteristics for research. Because the full width at half maximum (FWHM) of high-performance narrow-linewidth lasers is usually within tens of MHz, it is very difficult for them to directly meet the application requirements in the fields that need broadband source and give full play to their advantages in terms of cost and performance, which also limits the development of laser to a certain extent. If the laser is directly modulated to achieve linewidth broadening, it will cause serious frequency drift and destroy its important frequency stability. In order to obtain the wide spectrum without degrading wavelength stability, an external phase modulation method using Gaussian white noise (GWN) as a driving signal is reported. Based on the relationship between laser phase noise and spectral characteristics, the broadening effect of phase noise on the spectrum is clarified through theoretical calculation. The spectrum evolution process with different parameters of white noise is analyzed based on OptiSystem optical simulation tool and numerical simulation. The noise source with high power and wide bandwidth is needed for broadening and carrier suppression of high efficiency. The external phase modulation system uses a 10 GHz noise source. The signal is increased from -17 to 28 dBm by two preamplifiers and a high saturation output power amplifier to drive a lithium niobate (LiNbO₃) electro-optic phase modulator with the half-wave voltage of 3.7 V. The low-pass filter and variable attenuator are added to the amplification link to adjust the signal parameters. The research object is a distributed feedback (DFB) semiconductor laser with an initial linewidth of 20 kHz. It can be seen by observing and comparing the laser spectra with and without modulation using an optical spectrum analyzer (OSA) that the experimental results are consistent with the theory and simulation. The maximum broadened component is up to 65 GHz and the carrier is also obviously suppressed. In addition, the laser frequency drift is evaluated based on the scanning Fabry-Pérot confocal cavity. It is proved that the external modulation does not affect the wavelength stability of the laser, so the phase modulation scheme of GWN can achieve superior spectrum broadening. Because the modulated spectrum has no sideband or minor peak and the broadened component is several times the modulation bandwidth, GWN has obvious advantages compared with others, such as sinusoidal signal and pseudo-random binary sequence (PRBS) signal. This promising method does not depend on the intrinsic linewidth of laser, and the work can be used as a reference for the research of phase modulation and spectrum broadening.