The performance of Stimulated Brillouin Scattering (SBS) suppression is one of the key parameters in optical fiber AM-CATV (amplitude modulated cable television) externally modulated transmitters. The influence of the incident polarization state with TE mode and TM mode on the performance for SBS suppression in optical fiber AM-CATV externally modulated transmitters is analyzed theoretically. The analysis and calculation have shown that the TE mode is the best mode for performance of SBS suppression. The influence of the incident polarization state deviating from TE mode is also analyzed and calculated. And it is shown that the deviation angle from the TE mode should be less than 10 degrees so as to the performance for SBS suppression will not be degraded.