The spectrum characteristics of self-mixing signals observed in optical feedback self-mixing interferometry (OFSMI) is studied in this paper. The purpose is to provide guidance for the design of pre-processing techniques for eliminating noise or disturbance. The influence of two important parameters of the OFSMI system on the spectrum, that is, the optical feedback factor and the linewidth enhancment factor, are measured by means of Discrete Fourier transform (DFT). The simulated results show that, at a weak feedback, the OFSMI signals are strictly band limited in nature, with the cut-off frequencies being the vibration frequency and the fringe frequency respectively. Also increasing broadens the bandwidth but has little influence on the bandwidth of the signal. At moderate feedback, the OFSMI signals are still band limited with much higher cut-off frequencies, and there is a considerable spectrum spread over a very wide range in the high frequency end. The presented analysis provides sufficient information for applying signal processing to OFSMI signals such as the filter design.