Delicate control of the initial polarization states of sub-beams incident to the interferometer unit in multibeam Lloyd's mirror interference lithography is of great significance for the fabrication of high-precision two-dimensional (2D) scale gratings. In this work, the optimization of the initial polarization states of incident sub-beams is formulated as a constrained multiobjective optimization problem. A generalized method that allows the incident sub-beams to initially have the general elliptical polarization states in the optimization is proposed based on the multiobjective genetic algorithm. It is demonstrated that the method is capable of delivering ideal initial polarization states with which the cross-interference between the two once-reflected sub-beams on the grating substrate of the interferometer unit can be completely eliminated from the resultant 2D interference fringes. The feasibility of the proposed method is verified through the comparison between the simulated 2D interference fringes and the fabricated 2D scale gratings under the achieved ideal initial polarization states.