According to the fractal theory, soot aggregates which have typical morphologies coated with water are generated in high and low relative humidity conditions. The relationships among relative humidity of atmospheric aerosol, equivalent refractive index and radius of soot particle are investigated. It is found that the radius of soot particle coated with water remarkably increases with the increase of relative humidity. The radiation properties of soot particles are calculated by means of Maxwell-Garnett (MG) theory and T matrix (MG-T) method, while the accuracy of the calculation results is verified by Coat-Mie theory. The calculation results of the radiation properties of soot particles calculated by MG-T method are in well agreement with that by Coat-Mie theory. The radiation properties of two kinds of soot aggregates in infrared wavelength are calculated by MG-T method. The results reveal that the values of radiation properties of packed soot aggregates are much larger than those of the branched soot aggregates with the increasing relative humidity. When the relative humidity is larger than 50%, the values of the radiation properties of the two kinds of soot aggregates increase due to the increase of water layer thickness and radius of soot aggregates. It is confirmed that the radius of soot aggregates plays a critical role in determining the radiation properties for soot aggregates.