To determine the reaction pathway of unsym-dimethylhydrazine (UDMH) to nitrosodimethylamine (NDMA) in the atmosphere, the reaction mechanism of UDMH oxidation to NDMA in the atmosphere was studied using quantum chemical method. The geometric configuration optimization and frequency calculation of reactants, intermediates, transition states and products of the reaction system were carried out at B3LYP/6-311+G(d,p) and M06-2X/6-311+G(d,p) levels. The single points energies of the species were corrected at the CCSD(T)/aug-cc-pVTZ level and the potential energy surface profile for the reactions was constructed. Results show that the oxidation initiation of UDMH is a dehydrogenation or oxygenation process. Considering the concentration of oxidants in the atmosphere, the ratio of reaction rate of UDMH initiated by different oxidants is: v(O₃)≈10⁴×v(HO·)≈10⁸ ×v(O₂) .The oxidation of UDMH is mainly trigged by ozone, forming (CH₃)₂NNH(IM1) after the removal of hydrogen atoms. Then IM1 can be converted into NDMA under the action of HO·, HO₂·, O₃ in the atmosphere environment. Ozone participation in the process will reduce the activation energy of the reaction and promote the formation of NDMA in the atmosphere.