This paper presents a rotate speed sensor using fiber Bragg grating (FBG) as sensitive component and constant strength cantilever structure as elastomer. The sensor is applied in large rotating experimental platform testing changes of impeller rotate speed through the vibration of the sensor's elastomer. The center reflection wavelength of FBG enters fiber grating demodulation device sm130, where light signal is demodulated. Experimental results demonstrate that the sensor can test the rotate speed of rotating machine repeatedly and sensitively. In addition, the sensor's easy theory and portable structure lay the foundation for further study on shaft torsional vibration detection of rotating machinery.