Erbium-doped superfluorescent fiber source is widely used in medium- and high-precision fiber-optic gyroscopes. The mean wavelength of the output light drifts as its use time increases. Consequently, the scale-factor stability of fiber-optic gyroscopes is insufficient to satisfy the requirements for long-term application. To solve this problem, this paper proposes a technology to improve the long-term stability of the mean wavelength, which involves incorporating a pump-power control loop to the light source to stabilize the pump power and reduce the pump-power loss and mean-wavelength drift caused by the aging of the pump source. Optical-path filtering technology is adopted to filter the unstable band light in the spectrum to improve the spectral stability and symmetry, and the filter parameters are optimized via simulation. Experimental results show that the mean-wavelength stability of the fiber-optic gyroscope using this method improves by 88%, the scale-factor stability exceeds 5 ppm within 9 months at room temperature, and other parameters of the improved fiber-optic gyroscope are not significantly degraded, thus demonstrating the effectiveness of the method.