This paper describes the development of a new point kinetics model for sub-critical nuclear reactors, driven by an external neutron source of the ADS-type. The equations and kinetic parameters were obtained with the use of the importance function, associated to the fission rate, as the weight function. Based on the point kinetics equations of this new model it was possible to get a new definition for the multiplication factor for the ADS-type sub-critical system. The comparison of the results obtained by this new point kinetics model with those of spatial kinetics, for the ABI (Accelerator Beam Interruption) and ABO (Accelerator Beam Overpower) transients postulated showed outstanding agreement, its highest relative deviation being of −0.612%.