Abstract: The reaction mechanism of CH2Cl radical with OH radical to produce HCCl+H2O，HCOCl+H2 and H2CO+HCl has been studied by using quantum chemistry ab initio calculations. The optimized geometrical parameters，and vibrational frequencies of all species were obtained at the UMP2（FC）level of theory in conjunction with 6-311++G* basis set. Besides，the zero-point energies（ZPE），relative energies and total energies of all species were calculated using Gaussian-3（G3）model. The results of theoretical study indicate that the activated intermediate CH2ClOH is first formed through a barrierless process，followed by atoms migration，radical groups rotation and bonds fission to produce HCCl+H2O，HCOCl+H2 and H2CO+HCl，respectively. And all channels are exothermic by 72.81，338.54 and 354.08 kJ/mol. The reaction heat of reactants to H2CO+HCl is 281.27 kJ/mol more than that of reactants to HCCl+H2O. This result accords with that of experiments.