The stability of beat frequency in the double-longitudinal-mode He-Ne laser is discussed. Considering the mode-pulling effect, the relation between the stability of the single mode and that of the beat frequency is deduced theoretically. The stability of the beat frequency in 632.8 nm He-Ne laser is 1000 times of that of the single mode with the frequency pulling parameter of 10-3. The experimental results show that the stability of the beat frequency is 3.3×10－6, when the single-mode stability reaches 1×10－9.