By increasing the sample scattering layer thickness of laser guide star,the detection energy of wavefront sensor is enhanced but makes the detection spot diffused.To solve this problem,a dynamic focus system is designed.Influence of the sample thickness on the wave-front detection energy is analyzed and result shows that the wave-front detection energy increases nonlinearly with the sample thickness.When the sample thickness is 4 km with the laser pulse energy of 10 mJ,the number of received photons is 120 per sub-aperture of telescope,which meets the detection requirements.Then effect of diffusion spot on wave-front detection accuracy is analyzed,the result shows that the wave-front measurement error is about 0.5 λ when the sample thickness is 4 km,which leads accurate detection unfeasible.In order to achieve accurate detection,dynamic focus system is designed based on the analysis results,moving distance of the focusing mirror is reduced from 111 mm to 100 μm,and moving speed from 4200 m/s to 3.8 m/s,which meet the application requirements.Zemax softwave optimized results show that the optical system can achieve the ideal imaging for 4km sample thickness range.