The molecular geometries，electronic structures and IR spectra at the four conformations of pentaerythritol diazido dinitrate have been calculated by means of DFT-B3LYP method. Among the four conformations，the one with structure of one -NO₂ group at cis-position and other -NO₂ group and two -N₃ groups at trans-position is the most stable.The calculated absorbed frequencies of -NO₂ groups and -N₃ groups in IR spectra are good consonant with the corresponding experimental.