Soil erosion is a serious environmental problem closely associated with sustainable development and ultimately the survival of mankind. Cesium-137, a unique artificial radioactive tracer, has been widely applied to the study of soil erosion and deposition since the 1960s. Furthermore, it is a basis for determining a Cesium-137 Reference Inventory (CRI) that employs cesium-137 to measure soil erosion, which can directly influence the accuracy and reliability of the soil erosion rate. This paper references 102 CRI data samples collected from over 80 documents; it also uses the monthly precipitation dataset from the Global Precipitation Climatology Centre from 1981-2010, with spatial resolutions of 2.5°×2.5° and 0.5°×0.5°. The Modified CRI Model for the Mainland of China (MCM) that the paper established is based on incorporating and modifying two previous models, the Walling & He Model (WHM) and the Michio Aoyama Model (MAM). Then we calculate the geographical distributions of CRI by using Kriging/Cokriging interpolation. The model assessment and comparative analysis demonstrate that MCM simulated values are generally in agreement with the observed values and greater than WHM and MAM simulated values. MCM can be applied to higher resolution and higher precision CRI modeling in the Mainland of China. The results show that the range of CRI in the Mainland of China is between 141 and 12123 Bq/m², and the maximum values are found in parts of northeast China and Xinjiang regions. The minimum values generally come from the regions south of 25°N. Except for some parts of Xinjiang, distributions of CRI in the Mainland of China indicate that CRI increases with precipitation from west to east of the same latitude, while zonal distributions of CRI indicate that CRI increases with the increase of the latitude. Besides, other factors such as large-scale atmospheric flow field, re-suspension, and local nuclear testing contribute to the heterogeneity of CRI in the Mainland of China.