A new method of generating periodic linear chirped pulse is proposed, and a 0.52 nm bandwidth periodic chirped pulse is generated. Numerical simulation shows that the gain saturation, group velocity dispersion and synchronization jitter have great influences on chirped pulse; and self-phase modulation has little influence since the peak power stays at a low level. The experimental results indicate that the polarization mode dispersion will lead to a great intensity modulation, and the amplified spontaneous emission will break the light stability, which leads to the fewer round-trips of light. Research shows that the direct phase modulation is a novel method of generating periodic linear chirped pulse, which could be used for smoothing the spectral dispersion.