Effect of light spot size and optical axis deflection on heterodyne efficiency of space optical hybrid was focused on in this paper. With deduction of the heterodyne efficiency expression based on Gaussian model, the result shows that when the proportion of the light spot size and target plane equals 0.64, the heterodyne efficiency reaches optimal. Then by calculating and simulating the expressions which included spot size deviation and optical axis deflection, the curve of relation between heterodyne efficiency and spot size deviation and optical axis deflection was obtained. By these works several results can be shown: Firstly, the scope of spot size deviation is between -42 μm and 250 μm and the scope of axis deflection is 203 μrad when heterodyne efficiency is 10%. Besides, by calculating the spot size deviation and comprehensive optical axis deflection error, negative influence of heterodyne caused by axis error can be improved with modulating the light spot size deviation. In order to test the theoretical result, an experimental platform of influence factor of spatial optical hybrid was built up and an experiment of deflection angle of optical axis with choosing three light spot sizes was implemented. The result is consistent with that of the simulation. The curves of actual measurement are slightly less than the theoretical value because of the influence of insertion loss caused by experiment components. These influence obtained may provide technical reference for the design, adjustment and process of spatial optical hybrid.