Abstract: Fluence spectrum is required to calculate parameters such as average energy, resolution and conversion coefficient when filtered X ray reference radiation beyond ISO 4037 1 is to be constructed. Aiming to establish the method of obtaining fluence spectrum of filtered X ray, reference radiations from N 40 to N 250 were measured with N type coaxial HPGe detector and pulse height spectra were collected. The detector’s responses to several radioactive sources were simulated with Geant4 and verified by measured spectra. Then the response matrix was constructed, and fluence spectra were unfolded with MAXED software. The conversion coefficients from air KERMA （kinetic energy released in material） to ambient dose equivalent and personal dose equivalent were calculated with the unfolded spectra and compared with those recommended by ISO 4037 3. The results show that the relative deviations are within 0 06% 2 55%.