Periodic nanostructure arrays have attracted great interest because of their unique optical effects with great potential for applications in novel sensing technologies. Their optical properties depend on morphology and structural parameters, and their optical properties can generally be regulated by tuning these parameters, while the adjustment of their optical properties by an applied magnetic field has rarely been reported. We prepared colloidal crystal templates by gas-liquid self-assembly method and realized the regulation of the structure size of colloidal crystal templates by a plasma etching technique. Combined with magnetron sputtering technology, subwavelength-sized magnetic Co nanosphere array films with hexagonal periodic arrangement were synthesized, and its optical properties under the action of structural parameters and external magnetic field were investigated. The UV-Vis-NIR light reflection spectrum showed that the peak position of the light reflection peak shifted from 512 to 430 nm in the visible band with an increase in etching time from 0 min to 4.5 min, a blue shift of 82 nm. The peak intensity of light reflection decreased from 10.69% to 7.69%, weakening by 2.73%. In the NIR band, the peak position of the light reflection peak was blue-shifted by 237 nm f rom 1 929 to 1 692 nm, and the peak was reduced by 3.01% from 10.92% to 7.91%. By controlling the etching time, effective tuning of the peak position and peak intensity of the light reflection peak of the Co nanosphere array films can be achieved. A perpendicular external magnetic field was applied to both the unetched and etched Co nanosphere array films, and both exhibited different degrees of enhancement in the peak intensity of light reflection under the effect of the external magnetic field. The peak intensity of the optical reflection of the unetched Co nanosphere array films in the NIR band (1 938 nm) increased from 10.81% (0 Oe) to 16.56% (1 100 Oe) with the increase of the applied magnetic field, with an enhancement of 5.8%. Nevertheless, the NIR reflection peak (1 921 nm) of the Co nanosphere array films after plasma etching, its peak intensity increased from 8.45% (0 Oe) to 16.74% (1 000 Oe), which is enhanced by 8.29%. The results show that the reflection spectra of the magnetic Co nanosphere array films after plasma etching exhibit a more sensitive external magnetic field response. Based on the relationship between the maximum value of the NIR light reflection peak and the strength of the external magnetic field, the effect of the external magnetic field on the light reflection performance of the magnetic Co nanosphere array films was qualitatively explained. For unetched samples, the external magnetic field mainly changes the magnetic order of the sample, thereby affecting its complex refractive index and thus its light reflection properties. For the etched sample, in addition to the influence of the external magnetic field on the magnetic order of the sample, which affects its light reflectivity, there are other competing physical mechanisms such as scattering, diffraction and so on. This study provides a method for dynamically modulating the light reflection properties of materials by magnetic fields and a model for the study of novel optical devices.