A method for designing an optical device which includes a lens and a microlens array is disclosed. A point spread function (PSF) of the lens including rotationally symmetrical aberration coefficients is formulated, wherein the PSF presents various spherical spot sizes. A virtual phase mask having phase coefficients is provided and the phase coefficients are added to the PSF of the lens, such that the various spherical spot sizes are homogenized. The virtual phase mask is transformed into a polynomial function comprising high and low order aberration coefficients. A surface contour of the lens is determined according to the rotationally symmetrical aberration coefficients and the low order aberration coefficients, and a sag height of each microlens in the microlens array is determined according to the high order aberration coefficients. An optical device using the design method is also disclosed.