Abstract: A response function model for the XCounter Flite X1 photon counting detector was built. Using X-ray fluorescence as ray-source, the incident spectrum and the deposited spectrum were simulated by Monte Carlo method. Gaussian model and phenomenological model were used in the response function, while the parameters were determined by the least square method correspondingly. The inconsistency correction of the detector consisted of the threshold correction and the function correction. So that the error between the simulation result and experiment data was less than 2.5% in each energy bin. The spectral restoration and material decomposition result with the response function get much closer to the ground-truth, which shows the effectiveness and accuracy of the proposed method.