The beat frequency in a dual frequency He—Ne laser varies while the resonant cavity length is tuned.As to the laser with two longitudinal modes,the variation amplitude is commonly less than 500 kHz,proven by experiments and theories.This study reveals an anomalous variation of the beat frequency when a piece of element is put into the cavity and is aligned with the laser axis.Consequently the variation amplitude could reach 22 MHz,several dozen times larger than that without the intra-cavity element.This cannot be explained only by laser mode pulling and pushing effects.Some influencing factors are investigated experimentally,including the tilted angle of the element and the distance between its surface and cavity mirror.The qualitative analysis is discussed,which agrees with the experimental results. The beat frequency in a dual frequency He-Ne laser yet while the resonant cavity length is tuned. As to the laser with two longitudinal modes, the variation amplitude is commonly less than 500 kHz, proven by experiments and theories. This study reveals an anomalous variation of the beat frequency when a piece of element is put into the cavity and is aligned with the laser axis. Conclusion that the variation amplitude could reach 22 MHz, several dozen times larger than that without the intra-cavity element. This can not be explained only by laser mode pulling and pushing effects. Some influencing factors are found experimentally, including the tilted angle of the element and the distance between its surface and cavity mirror. The qualitative analysis is discussed, which agrees with the experimental results.