Spatial narrow linewidth laser, as the local oscillation of the spatial strontium atomic clock, determines the short- and medium-term stability of the space strontium atomic clock. Due to the limited load resources of the space station, the optical system of spatial narrow linewidth laser not only meets the functional requirements of the space light clock reference transition, but also considers the factors of volume and weight. In addition, the space narrow linewidth laser system has strict requirements on the power and heat consumptions. Improving the optical power utilization efficiency under the condition that the output optical power of the laser source is constant is the key to ensure the follow-up optical path power demand. In order to reduce the volume and the weight of the system, we build the optical path of the system with the miniaturized optical components. To improve the utilization ratio of the optical power of the system, we build the experimental platform of the optical functional unit on the basis of the short focal two-lens scheme. The result shows that the single-pass diffraction efficiency of the acousto-optic modulator is greater than 90%, and the double-pass diffraction efficiency is greater than 70%. We scan the acousto-optic modulator drive signal. It is found that the optical fiber coupling efficiency changes by 50% within ±20 MHz scanning range, which satisfies the space and power application requirements.