The reactions of HOCl with HCl on water clusters have been theoretically investigated. Ab initio calculations indicate that hydrogen bond and cooperative effect play an essential role in the reactions; that the reaction barrier of HOCl with cyclic (H2O)(n). HCl cluster is the least at n=3 and that the ionization of HCl and HOCl on ice surfaces may not be complete but partly at very low temperatures. Two cases a and b of the model reactions are considered for detailed analysis. On the surface of ice, the barrier energies are about 4 and 6 kcal/mol for cases a and b, respectively, at the MP2//HF/6-31G(d) level, which is close to an experimental estimation. This study suggests a similar previously reported mechanism that the heterogeneous reaction of HOCl with HCl on ice is catalyzed at the stratospheric conditions through structure catalysis and hydration that enhances ion character of species.