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Photosynthetically Available Radiation (PAR) is an important bio-optical parameter related to marine primary production. PAR is usually measured by a broadband sensor and can also be calculated by multispectral data. When the PAR is calculated by multispectral data in polar region, four factors are possible error sources. PAR could be overestimated as the wavelengths of muhispectral instrument are usually chosen to evade main absorption zones of atmosphere.However, both PARs calculated by hyperspectral and muhispectral data are consistent with an error less than 1%. By the fitting function proposed here, the PAR calculated by multispectral data could attain the same accuracy with that by hyperspectral data. To calculate the attenuation rate of the PAR needs PAR0,the PAR just under the surface. Here, an approach is proposed to calculate PAR0 by the best fit of the irradiance profile of 1-5 m with a content attenuation coefficient under surface. It is demonstrated by theory and observed data in different time at same location that the attenuation coefficient of PAR is independent of the intensity of radiation. But under sea ice, the attenuation coefficient of PAR is a little bit different, as the spectrum of the light has been changed by selective absorption by the sea ice. Therefore, the difference of inclusions inside the sea ice will result in different PAR, and impact on the attenuation of PAR. By the results of this paper, PAR can be calculated reliably by multispectral data.