A semi-empirical model for ion mixing at low temperatures was developed taking into account collisional mixing and thermal spike effects, as well as the thermal spike shape. The collisional mixing part was accounted for by the Kinchin-Pease model, or, alternatively dynamic Monte Carlo simulation. For the thermal spike, the ion beam mixing parameter Dt/Φ is derived as being proportional to χ2+μ, where the damage parameter is defined as, χ = − F0/ΔHcoh, F0 is the damage energy deposited per unit path length, and μ is a constant dependent on the thermal spike shape and point defect density in the thermal spike regions. The shape of the thermal spike that best fit the experimental results depends on the magnitude of the cascade density. For relatively high density collisional cascades, where thermal spikes start to be important, it was found that a spherical thermal spike model was more consistent with experimental measurements at low temperatures. However, for extremely high density collisional cascade regions, a cylindrical thermal spike gave better results. Finally, three different regions of ion beam induced mixing were recognized according to different density levels of damage energy scaled by the damage parameter χ.