The present invention provides a radiation measuring device for confirming the relevant characteristics of radiant wavelength from radiation source. The radiation measuring device includes a wavelength-related optical component (e.g. band-pass filter) and an optical power measuring detector (e.g. optical detector). At least a light beam enters into a polarization-sensitive reflection and/or transmission surface arranged along the light path of the device, which is transmitted by the wavelength-related optical component and received by the optical power measuring detector of the light path. Theradiation measuring device also includes a linear polarimeter arranged before the optical power measuring detector along the light path. In operation, the linear polarimeter ensures the light beam received by the polarization-sensitive reflection and/or transmission surface is basically under fixed polarization state which has nothing to do with the polarization state of the original light beam,thus reducing or eliminating the uncontrollable polarization-related error in the radiation measuring device.