Because measured objects are far away, it is difficult to establish a high accurate common measurement reference in the measurement of large spatial angle. In order to achieve the large-scale spatial angle measurement conveniently, a novel large spatial angle measurement method based on inertial reference was proposed. Firstly, the measuring principle of large spatial angle measurement was explained in detail and the measurement system was designed. Meanwhile, the pointing uncertainty of optical axis based on the two dimensional galvanometer in measurement system was studied and the influence of different errors on optical axis pointing was analyzed. Finally, the pointing uncertainty of optical axis caused by different errors was evaluated using Monte Carlo simulation. The research can lay a foundation for error allocation and pointing accuracy evaluation of the measurement system.