The interference of space beam in gain medium generates space burning hole phenomenon, and forms the gain grating. The gain grating formed by the four-wave mixing function has adaptive, self-tuning Q and spatial filtering capability, and can obtain holographic conjugate output. But all of them start from the spontaneous emission in the cavity. Spontaneous emission is easily influenced by the pump, and its bandwidth is wide, which will cause a certain fluctuation in the output frequency. In this paper, a single-frequency Q-switched pulse output locked in the seed spectrum range is obtained by an external single-frequency narrow linewidth source injection in a self-starting single-frequency laser based on a gain grating. Comparative experiment shows that in the same observation time, the spectral stability of the injected output is much higher than the self-starting output. This phenomenon also proves that the steady frequency output can be obtained by the adaptability of the gain grating formed by external seed injection.