Hemispherical resonator gyros are widely used for spacecraft attitude control. In this study，the noise composition of a hemispherical resonator gyro was analyzed using Allan variance and compared with that of fiber-optic and MEMS gyro. The results of the Allan variance analysis showed that the noise of the hemispherical resonator gyro is primarily quantization noise，and the component of the angle random walk is very small，whereas the noise of the fiber-optic and MEMS gyroscopes is composed of an angle random walk and basically no quantization noise. Therefore，the quantization noise should be reduced to reduce the noise of the hemispherical resonator gyro. By adjusting the circuit parameters，the noise of the gyro was reduced，which proves the effectiveness of the measures and indicates a direction for further reducing the noise of the hemispherical resonator gyro.