In order to reduce the dielectric effects, mainly induced by the reflection of infrared signals and the transmission of electromagnetic waves, on the electric field phases of the infrared/millimeter-wave co-aperture object simulator and improve the direction uniformity of infrared and millimeter-wave signals, the geometrical optics (GO) method was combined with the genetic algorithm to obtain the shape functions for generating the dielectric surfaces. The amplitude and phase distributions of the electric fields at the specific distance from the optimized co-aperture object simulator were computed by using the commercial EM software. The numerical results illustrate that the phase variation of the electric fields of the co-aperture object simulator is very small after optimization, and good direction consistency of infrared and millimeter-wave signals is also observed.