The Modulator′s bias point setting is very important for optical fiber AM CATV externally modulated transmission. In this paper it is concluded that when the bias point drifts away from the best bias point, CSO (Composite Second Order distortion) is influenced severely, whereas CTB (Composite Triple Beat distortion) is not influenced. The related formulas were deduced for PAL/D 550 MHz CATV transmission, the maximum voltage deviation from the best bias point is less than 10 mV. The experimental results agree well with the theoretical analysis.