As an important thermal detection material, Mn-Co-Ni-O has been widely used in variety of fields as the key thermal detector. NiMn₂O₄ and Mn_1.56Co_0.96Ni_0.48O₄ bulk materials were prepared by grinding and wintering methods. The crystallization of the two blocks was studied by X-ray diffraction experiments, and it was found that the addition of Co cation with larger radii resulted in deterioration of the bulk crystalline. The optical constants and dielectric constants of NiMn₂O₄ and Mn_1.56Co_0.96Ni_0.48O₄ in the ultraviolet-far infrared wide band were obtained by ellipsometry. It was found that significant difference appeared between the optical properties (intensity and peak position of optical constants) of the two materials after the addition of ions. The reflectance spectra of the two materials were obtained by Fourier spectrometer, it was compared with the calculated values of optical constants, so the effect of surface roughness on reflection spectra was evaluated.