Abstract: This study describes a method to reconstruct energy spectra of clinical bremsstahlung X-rays from measured depth dose data. The measured depth dose can be expressed as a linear combination of basis functions, namely the depth doses of monoenergetic photon beams derived from Monte-Carlo simulations. The weights of the basis functions, which were obtained with the Cimmino iteration, represent the relative energy spectra. The real energy spectra are changed gradually with the off-axis distance because of the photons scattered from components of treatment head. The energy spectra of X-rays in the isocenter plane were reconstructed using a dual-source model in three regions: 5 cm×5 cm, 10 cm×10 cm-5 cm×5 cm, 20 cm×20 cm-10 cm×10 cm. Depth doses produced from the uniform spectrum (10 cm×10 cm) and from regional spectrum were compared with measured depth doses respectively. The results show that using the dual-source model in reconstructing spectrum is more appropriate in clinical, and depth doses produced from regional spectrum are more identical to measured depth doses.