Abstract: The effect of composition ununiformity of LPE HgCdTe film on spectral response of device was studied. A new method was proposed to calculate the spectral response of HgCdTe IR detector. This method considered longitudinal composition distribution and transverse composition ununiformity in LPE HgCdTe film. The coherent and incoherent light transmission among the device was also included. Using this method, the variation law of peak responsibility of spectral responseand and cutoff wavelength with compositional diffusion width z and transverse composition deviation of LPE HgCdTe film was calculated. The result indicates that for common LPE HgCdTe film, whose is less than 0.002, the influence of transverse composition ununiformity on spectral response can be neglected. The changes of peak responsibility and black body responsibility with thickness of LPE HgCdTe film were also calculated. The optimum thickness can be obtained.