The typical size-of-source effect results of radiation thermometers show that verification result differences between different measurement conditions are possible to be several times of the absolute value of the maximum permissible error of the thermometers. A plane radiation source ( PRS) model for aiming of the thermometers with a definitive measurement condition is presented, the radiation source-preposed aperture method to realize a unique equivalent PRS by the radiation sources with the different cavity sizes, and the technical characteristics and placed distance requirement are also recommended. Analysis shows that an inappropriate cooling from a low temperature radiation source to the aperture leads a measurement result drop which is not neglectable. The experiments adopted the equivalent PRS show the consistent verification results obtained by different-characteristic radiation sources. The problems which would be faced up to when the PRS model is applied and measures are discussed.