The proton transfer in NH3-HCl by only one molecule of catalyst was studied by using the MP2 method with the large 6-311++G(2d,2p) basis set. The IS structures are obtained for the smallest units. NHE3-HCl-A trimers, for which the proton transfer maybe occurred. The final results show that the proton transfers have occurred in the 15 cyclic shape structures for A = H2SO4, H2SO3, HCOOH (a), HF, H2O2, HNO3, (a), CH3OH, HCl, HNC, H2O, HNO2, (b), NH3, HCOOH (b), and HCHO, and not occurred in another 3 trimer structures for A HCN, H2S, and PH3. These results show that the proton transfer occurs from HCl to NH3 when catalyst molecule A (acidic, neutral, or basic) not only as a proton donor Strongly donates the proton to the Cl atom but as an acceptor strongly accepts the proton from the NH3 molecule in the cyclic H-bond structure. In this work, a proton circumfluence model is proposed to explain the mechanism of the proton transfer. We find that, for the trimer, when the sum of two hydrogen bond lengths (R = R-1 divided by R-2) is shorter than 5.0 Angstrom, molecule A has the ability to catalyze the proton transfer. In addition, we also find that the interaction energy E-int between NH3-HCl and A is nearly related to the extent (RHl-Cl) of proton transfer, that is, the interaction energy E-int increases with the proton transfer.