Based on the homemade frequency-swept laser source with a narrow instantaneous line width, the main factors influencing the laser instantaneous line width are investigated by numerical model and experiment. The research results indicate that the instantaneous line width of the frequency-swept laser increases with the increment of dispersion, and the filter bandwidth and the signal pattern loaded to the filter both influence the instantaneous line width. At the same time, the intra-cavity mode competition can ensure a narrower instantaneous laser line width.