We theoretically study the spin-polarized electron transport properties of a quantum wire under irradiation of a transversely polarized external electromagnetic (EM) wave in the presence of Rashba spin-orbit interaction (SOI). Using the free-electron model and the scattering matrix approach, we show that spin polarization is independent on the EM frequency and the electron incident energy when Rashba SOI is weak, while it is strongly dependent on the EM frequency and the incident energy when Rashba SOI is comparatively strong. In addition, an unusual step-structure appears in the electronic transmission probability as the incident energy increases to a threshold value when a coherent EM field is applied. We demonstrate that the first effect comes from Rashba subband intermixing and the latter comes from inelastic scattering of electrons with lateral photons through intersubband transitions.