LCAC-SW method has been extended to study the reaction dynamics for ion-pair formation processes. M+ X2→-M+ + X2- reaction system involves two potential energy surfaces, i.e., the covalence state (M+ X2) and the ionic state (M+ + X2-) and their crossing effect. The working equations for calculating state-to-state probability have been derived based on the above two-state model. Satisfied results of collinear state-to-state probabilities for K+ I2→K+ + I2- ion-pair formation system have been obtained.