Abstract: In this paper, we have compared the characteristics of two types of chromatic coherent light sources from both ways of analytical analysis and numerical simulation. The results show that comparing coherent harmonics generated by fundamental mode bunching in a mormal resonantor and by external laser prebunching in an optical klystron amplifier, the output power of the high harmonics of the former is the same order of magnitude with the latter, except more one fundamental mode output than the latter, and the former does not need an external source and only need research the reflectivity of mirrors and the. power extract for the lights with different frequency.