An optical bandpass filter (OBF) is an important element whose properties considerably influence the output performance of a single side band multicarrier source based on a re-circulating frequency shifter. The influence of the OBF in the loop on the output spectrum is theoretically analyzed. Numerical simulations and experiments are also carried out. Results show that the steepness and deepness of a nonflat-top filter influence the stability of the spectrum of an output multicarrier. To obtain multicarrier output with tone to noise ratio (TNR) >26 dB and error vector magnitude (EVM) <0.25, the steepness ratio of the filter should be greater than 0.75 and deepness should be larger than 0.99.