In the design of multispectral infrared detectors, multispectral filters are key components for realizing focal plane integrated spectrometry. The wider the spectrum splitter and the farther the mounting position is from the detector, the worse the assembly response non-uniformity and the greater the crosstalk between spectral bands. Based on a 6-band 512×3×6 assembly, a design method for multispectral filter parameters and detector size is proposed through light simulation calculation. A set of calculation programs that can iteratively calculate and optimize the above parameters is designed based on the light simulation calculation logic. A filter mounting process method is proposed, which provides a useful solution for the optical design of subsequent similar assemblies. The multispectral infrared detector filter designed in this paper has a splitter width of 30 m, a thickness of 0.6 mm, and a distance of 0.9 cm from the detector; the pixel size of the detector is 40 m×40 m, and the pixel pitch is 900 m. The assembly has completed preparation verification, the cooling time is 416 s, the non-uniformity of the response rate in the six spectral bands are 10.9%, 12.5%, 11.8%, 11.9%, 12.3% and 10.9% respectively, and there is no crosstalk between adjacent spectral bands.