Among the theoretical and experimental research of the reflective intensity-modulated fiber optic sensor (RIM-FOS), a very important problem is always neglected, which is the effect on the intensity modulation caused by perpendicularity between the axis of fiber pairs and the reflector, i.e., the obliquity of the reflector.In the light of this, the mathematical analysis of this effect is performed.The simulation results of the RIM-FOS characteristic parameters with the different obliquity of the reflector are computed and the characteristic curves are plottedResults show that the obliquity of the reflector exerts great influence on the modulation function of the fiber pair.So advice is presented that the axis of fiber pair should be perpendicular to the reflector.