A reflective intensity modulated（RIM）fiber optic sensor is introduced for the rotation axis measurement of superconducting rotor in cryogenic instrument.A diffuse reflective rectangle pattern etched on the end face of the rotor is used to modulate the reflective optical intensity on which the quantum and direction of the rotation axis deviation is determined.Precision of pattern and size of facula which influences signal-obtaining are analyzed from the output waveforms which are simulated with Matlab.The result is helpful for the design of the fiber optic sensor and gives some useful reference for improving signal-obtaining precision in the application.