The characteristics of intensity noise of laser due to signal wave feedback in a fiber-optic gyroscope is investigated. It has been found that the signal wave feedback would cause a large amount of intensity noise of laser and that an intensity noise peak appears at the signal detection frequency. By establishing a theoretical model for the laser in a fiber-optic gyroscope,the relations of the intensity noise with various factors in the gyroscope are analyzed and the the results are in good agreement with those of experiment.Finally,the measured to reduce the effects of dignal wave feedback on system performances are presented.