In this paper, aiming at the feasibility of the visible and infrared large-aperture membrane diffractive optical systems in space application, we present a simulation model and fast analysis and calculation method of large-aperture diffraction optical imaging characteristics based on the two-dimensional finite difference time domain. First, we fully consider the electromagnetic field modulation characteristics of the subwavelength microstructure size of the large-aperture diffractive optical element, and establish the imaging characteristics analysis model of the sub-wavelength structure size diffractive optical system. Then, through vectorization programming and GPU acceleration calculation, we propose a fast simulation calculation method to solve the problem caused by vector calculation. Finally, we take a large-aperture diffractive optical system as an example and verified the imaging characteristics using the established imaging characteristics analysis method. Experimental results reveal that the proposed method can effectively characterize the imaging characteristics of the subwavelength microstructure diffractive optical system, and realize a rapid analysis of the imaging performance of diffractive optical elements.