Influence of the incident polarization state drifting off TE mode on composite second order distortion (CSO) and composite triple beat distortion (CTB) of an optical fiber AM CATV externally modulated transmitter is analysed theoretically. The conclusion is that if the insertion losses of the TE mode and TM mode are same and 59 PAL D channels are transmitted, the deviation angle from the TE mode should be less than 6 degrees with CSO=-70 dBc. The result of a single channel transmission experiment is consisistent with the theoretical analysis.