Abstract: Hardening trends of X-ray beams from three X-ray machines of different energies traversing aluminium plates of different thickness were simulated by means of Monte-Carlo method, as well as the processes of areal density measuring，and corresponding experimental researchwas done.The effects of X-ray beams hardening on the measurement sensitivity were analyzed. For the X-ray beams from X-ray machine the same energy, X-ray beams harden more with the areal density increasing, which brings about the reduction of the measurement sensitivity;The lower the energy of photons is , the more quicklythe measurement sensitivity reduces, and vice versa. For a certain areal density range, when X-ray photons can just traverse the part of the maximum areal density by adjusting the X-ray tube voltage, the best measurement sensitivity can be obtained.