In the low temperature environment generated by supersonic flow in the process of laser assisted retardation of condensation of isotope separation for BCl ₃, the molecular isotopes BCl ₃and carrier gas (rare gas (RG): He, Ne, Ar, Kr, Xe) can form BCl ₃:RG dimer via contact collision process. The mechanism and relationship between dimer concentration and absolute temperature of dimer involving BCl ₃molecules are of great significance for regulating and selecting the isotope separation parameters. In this work, based on the analytic description of the anharmonic interaction potential function of BCl ₃:RG, and considering the two-body and three-body collision induced association and dissociation of dimers, the concentration of BCl ₃:RG dimers is obtained at the absolute temperature in a range of 20–40 K. The obtained results are as follows. The two-body collision is dominant in the formation of dimer in the low temperature range. When the initial molar fraction of BCl ₃is in a range of 0.01–0.10, the BCl ₃:RG dimer concentration changes approximately linearly with the initial molar fraction of BCl ₃, indicating that the initial molar fraction not only determines the theoretical upper limit of the dimer concentration, but also dominates the dimer concentration in a low temperature range. When the temperature of the supersonic flow chamber is about 20 K, the concentration of BCl ₃:Kr dimers is largest, and the concentrations of other dimers are also presented. Furthermore, we explain the mechanism of laser assisted retardation of condensation in separation of isotopes by using a simple model on a molecular scale by adjusting the parameters of dissociation energy and stretching vibration frequency of the dimer.