Taking the ring assembly plane of cylinder part of coaxial optical system as the research object, the principle of tilt error transfer and compensation of fiducial axis in the case of multiple parts assembling was studied. Firstly, optical alignment of coaxial optical system and typical centering fiducial axis transfer paths were analyzed. And then error measurement and Least Square Method were conducted to fit the ring assembly plane of cylinder part and to get the coefficients of least-squares plane equation. By the theory of 3D geometry transformation, the transformation matrices precisely describing the tilt error of bottom face and top face in a single part, the transformation matrices of two coordinate systems of assembly joint surface in the case of two parts assembling and the function relationship between the angle between two Normal Lines of Initial Bottom Surface and End Top Surface (ANLIBSETS) and the Relative Load Angle (RLA) in the case of multiple parts assembling were established. Aiming at different types of centering fiducial axis transfer paths and applying different genetic algorithm plans, the optimization of ANLIBSETS was achieved. The method of solving the error of RLA was proposed. Finally, an optical alignment method of coaxial optical system based on global optimization alignment method was proposed. The feasibility and practicability of this method are verified by an example of a coaxial optical system.