Based on the dispersion vector analysis method of Buchdahl model,this paper designed a multi wavelength achromatic laser emission lens that can be used for laser ranging,which has low cost,small size,good beam quality and resistance to laser damage at peak power. Firstly,the dispersion vector of commonly used domestic high intensity laser lens materials was calculated by Buchdahl dispersion coefficient equation. Then,light tracing was performed on the optical system and the scale factor was calculated to calculate the Buchdahl dispersion vector of the non focal system. Finally,the optimization was guided by this result. The color difference of the optimized system at 0.707 aperture of 1064 nm,1572 nm and 1550 nm was only 2.49×10 -6D and 4.43×10 -6D,respectively. The color difference between 1550 nm and 1530 nm is only 3.04×10 -6D,the maximum color difference in the band range is only 4.43×10 -6D,and the maximum shift difference is only 0.0005 D. MTF and dot plot of each field of view fully reach the diffraction limit,and the polychromatic difference of the system is effectively corrected. The research results make the dispersion vector analysis method of Buchdahl model successfully applied to the focus free optical system in engineering design,and provide a new idea for the design of focus free optical system.