Abstract: Soil redistribution can be quantified by fallout radionuclides (such as ¹³⁷Cs and ²¹⁰Pb_ex) in many areas of the world in recent years. However, the time of abandonment is unknown, due to a large number of the abandoned farmlands. In this study, a dating model was proposed to accurately evaluate the farmland abandonment time in soil profile using ²¹⁰Pb_ex depth distribution. The characteristics of ²¹⁰Pb_ex was extremely sensitive to the soil environment. Firstly, a theoretical model was developed to determine the variations in the ²¹⁰Pb_ex depth distribution for the abandoned farmland. The redistribution process of ²¹⁰Pb_ex was then dissected after farmland abandonment. In addition, the exponential function of the ²¹⁰Pb_ex profile distribution was fitted in the abandoned farmland soil, according to the measured ²¹⁰Pb_ex depth distribution data. There was consistent with the form of the theoretical model. Moreover, there was the coefficient relationship between the theoretical and measured data in the farmland abandonment time from the ²¹⁰Pb_ex profile distribution. The dating model was derived for the farmland abandonment time. The field test was carried out to verify the reliability of the dating model. The typical sampling plots were selected as the three farmlands abandoned time for 19, 15 and 8 years in the lower reaches of the Jinsha River basin. Three sampling points were set randomly in each sampling plot. Specifically, a core was collected using an 8.0 cm diameter hand-operated corer at each sampling point. The sectioned cores were also collected using the steel core tube that inserted to a depth of 45 cm and then sectioned using 3 cm depth increments. The individual depth increments were bulked from the three cores to provide a single composite sample for each depth increment. The composite sectioned cores were used to determine the ¹³⁷Cs and ²¹⁰Pb_ex activities in the different soil layers. The age of the farmland abandonment was calculated using the newly-developed dating model with the measured ²¹⁰Pb_ex profile distribution data. The simulated values were used with the actual ages of the farmland abandonment to verify the accuracy of the developed model. The results showed that the farmland abandonment time by the dating model was in good agreement with the actual age. The dating accuracy of the model was higher with the increase of the farmland abandonment time. In particular, the farmland abandoned dating of 19 years was very close to the actual age, indicating the reliable dating model. This dating model was independent of the ground cover, location, and environmental factors. There were the high accuracy and stability of the farmland abandonment time. The application can be extended into the more and longer series of the abandoned farmlands in a wider range of areas. The dating model can provide the vital information to evaluate the soil erosion of the abandoned farmlands using the ²¹⁰Pb_ex tracing. The finding can also share an important reference for the impact of the farmland abandonment on the ecological environment.