Abstract: In order to solve the power fluctuation problem of 3kW RF slab CO₂ laser at high power operating conditions, a turning mirror adjusting method of out light path was adopt to compensate the light beam deviation caused by thermal deformation of cavity mirrors. The key technology of laser input power control by pulse width modulator based on TMS320LF2407 chip was studied. End mirror sampling power detection system of shaping light path was designed. And the compensation theory of out light adaptive regulation system was analyzed. The result shows that, the end mirror sampling laser power detection system of shaping light path has high linearity and the maximum error compared with reference power meter is ±2.59%. when using a piezoelectric ceramics microdisplacement driver of PA 100/T 14 to adjust and compensate the adaptive turning mirror with micro-motion distance less than 54μm, the light beam deviation will be less than 0.8mm of slit width of spatial filter. After adjustment and compensation, the output power fluctuation declines from 18% to 2%, and reaches the compensation demand of beam deviation caused by thermal deformation of cavity mirror. The sidelobe of shaping light disappears and the best output mode is gotten. The research has the practical value for improvement of laser stead output power.