The existing Pb-210(ex) mass balance models for the assessment of cultivated soil erosion are based on an assumption that Pb-210(ex) is quite evenly mixed within the plough layer. However, the amount of Pb-210(ex) distributed in the soils below the plough depth, like a downward tail in the lower part of the Pb-210(ex) profile, has been largely ignored. In fact, after the initial cultivation of undisturbed soils, Pb-210(ex) will diffuse downward from plough layer to the plough pan layer due to the concentration gradient. Assuming Pb-210(ex) inventory is constant, the depth distribution in the two layers of the cultivated soils will achieve a steady state after continuous cultivation for 10.37 years, when Pb-210(ex) is evenly distributed in the soils of the plough layer with an exponential concentration decline with depth in the soils of the plough pan layer, and the Pb-210(ex) concentration at any depth will be invariable with time. The work reported in this paper attempts to explain the formation of the Pb-210(ex) tail in the soil profile below the plough depth by theoretical derivation of the Pb-210(ex) depth distribution process in the two layers of the cultivated soils, propose a Pb-210(ex) mass balance model considering Pb-210(ex) diffusion based on the existing model, and discuss the influence of the Pb-210(ex) tail to the existing model.