The laser source is an important element whose properties have influence on the output performance of single sideband (SSB) optical comb based on recirculating frequency shifter (RFS). The theoretical and experimental results show that the tone-to-noise ratio (TNR), flatness, and stability of optical comb improve using laser source with narrower linewidth. In order to obtain the optical comb with TNR larger than 42 dB and degree of stability (DOS) smaller than -6.7 dB, external cavity laser (ECL) with linewidth from 100 kHz to 1 MHz is a trade-off choice.