In this work, the expressions of molecular dissociation rate and isotope enrichment coefficient in an strong IR field, as the functions of the laser energy per pulse, the focal length of lens, the number of laser pulses, and etc., were derived by using the kinetic theory. The theoretical expression of isotope enrichment coefficient was compared with the experimental results for SF6, and good agreement was found. In addition, the theoretical expressions compared with experiments would provide some useful informations of the molecular dissociation in the strong IR field.