The effect of LPE HgCdTe longitudinal composition distribution on device’s spectral response was studied. A new model was proposed to calculate the spectral response of HgCdTe IR detector. This model considered the real composition distribution in HgCdTe film. The coherent and incoherent light transmission among the device were also included. The calculated result indicates that the composition gradient of LPE HgCdTe film and the built-in electric field can evidently improve the response of the device. The calculated result agreed with the measured data, and proved the accuracy of the model.