Considering the optical focusing system in need of compact structure, the small focus spot size and avoiding the plume pollution in laser ablation micro thruster, the traditional ray tracing method was used to study the two typical optical design methods of single lens direct focusing and double lens for collimation and focusing. In the condition of fiber core diameter and numerical aperture for the low power semiconductor laser output and the design requirement of 50 micron focus light spot, the relationship between the design parameters of the lens thickness, the focal length and the working distance was obtained for the single lens focusing system. On the other hand, the relationship between design parameters such as lens thickness, lens center distance and working distance were also discussed, in the light of the design scheme of the double lens system for collimated focusing optical system. According to two typical schemes, the engineering application design parameters were given. The design method proposed in this paper avoids the complicated optical design process and provides a simple and practical method for the laser beam microscale focusing.