Linearly frequency-swept laser source is widely used in high spatial resolution, precision and dynamic range light measuring and optic fiber sensor. However, the frequency sweep nonlinearity and the phase noise of the laser source limit the system spatial resolution, precision and dynamic range. The mechanism of the nonlinearity and phase noise in frequency-swept distributed feedback (DFB) semiconductor laser is analyzed, and propose the combination of pre-distortion and optical phase-locked loop to overcome the nonlinearity of the frequency sweep and suppress the phase noise of the laser source. Chirp of 50 GHz is achieved where the root mean square frequency error is less than 263 kHz. And the performance of the frequency- swept laser source linearity and coherence enhancement is demonstrated.