A premise of X-ray fluorescence analysis is the determined acquisition of the primary energy spectral distribution emitted by the X-ray tube, and the energy spectral distribution function affects the accuracy of the final measurement greatly. An indirect approach to determine the primary energy spectral distribution of X-ray is derived, which includes relaying on the X-ray fluorescent intensities measured from the pure element sample of the thick target, establishing nonlinear functions with the known theoretical formula and obtaining the parameters in the parameter model through optimization. The comparison of the fluorescent intensity of the elements measured in the experiment and the theoretical value calculated with the energy spectral distribution function proves the approach valid.