In this paper, we propose our observation, analysis, and alleviation method of the beat frequency fluctuation in short-cavity dual-polarization distributed Bragg reflector (DBR) fiber grating laser sensor in time and frequency domain. According to our observation, beat frequency fluctuation varies from 8 to 800 kHz under different pump power. Research shows frequency domain beat signal linewidth broadening aggravates beat frequency fluctuation due to time domain irregular self-pulsation effect. By introducing self-injection into laser cavity as an auxiliary pump laser with a circulator, self-pulsation effect in DBR fiber laser sensor can be greatly suppressed, resulting in the compression of frequency domain beat signal linewidth as well as the stabilization of beat frequency fluctuation. Experimental results demonstrate that beat frequency fluctuation can be greatly reduced from 800 to 8 kHz within 1 h through self-injection, which is significant for high-accurate sensing.