In the field of precision measurement, HeNe laser is the first choice for fabricating laser interferometer. But the frequency difference of Birefringence-Zeeman dual-frequency HeNe laser, which can produce a medium frequency difference of 440 MHz, is randomly from as small as hundreds of kHz to as large as a dozen of MHz. To meet the production requirements for frequency difference of dual-frequency laser interferometer, there are some methods such as elastic strength and hole-drilling stress adjustment. But in the practical application, these methods are not very satisfactory. Laser-micro-engraving method assigns frequency difference by engraving a pattern in the plane mirror, and causes the phase retardation to achieve the purpose of frequency difference assignment. Laser engraving technique has many advantages: harmless to the laser, beauty, low power loss. The output power is close to the original light intensity. The frequency difference can be repeatedly adjusted and is stable. By frequency stabilization, the frequency difference fluctuates in a range of less than 10 kHz per hour.