With the rapid development of modern society, energy shortage and environmental pollution are becoming increasingly serious. Nowadays, investigations on new energy and new energy technology are commonly listed as a primary energy strategy worldwide. As a clean energy, solar energy is enormous in amount and the utilization of solar energy and related technology has attracted widespread concern around the world. Photovoltaic and photovoltaic heat technology can be coupled with nanofluid-based solar direct absorption thermal collector, which is an important means to improve the efficiency of comprehensive utilization of solar energy. Because nanofluids radiation theory play an important role in the development of new photovoltaic-thermal experimental platform and study on radiation characteristics of nanofluids is still in the initial stage, it is of great significance for the study of the law and mechanism of nanofluids radiation. In this paper, firstly, recent researches of nanofluid radiation characteristics were reviewed. The radiation characteristics of nanofluids were investigated theoretically and then the Rayleigh scattering model and the Mie model were used to analyze the critical radiation characteristics—transmittances of the nanofluids. Furthermore, the consistency between different theoretical models and experimental datum was studied by contrast verification between experiments and theory calculation. The conclusions showed that Mie model performed better than Rayleigh scattering model and express a better applicability in the development of photovoltaic thermal experimental platform. The purpose of this paper was using nanoparticles to change the radiation characteristics of the fluid while exploring a simple and efficient calculation criterion of the nanofluid radiation characteristics in practical design. Besides, the principle of the volume fraction which is one of the important factors for nanofluid radiation characteristics was obtained. Consequently, solar energy utilization rate of nanofluid-based solar direct absorption thermal collector would be improved. This theoretical study on radiation characteristics of nanofluids was expected to prompt on the application of nanotechnology in the field of solar energy and improve the efficiency of comprehensive utilization of solar energy.