Phosphorus is a key limiting factor for the eutrophication of water bodies. The phosphorus migrating from farmland soils is commonly recognized as the main source of phosphorus in water bodies. Therefore, a better understanding of the loss risk and its spatial uncertainty of soil phosphorus are important for the decision maker to regulate the loss of soil phosphorus at the local and regional scale. In this study, a total of 259 farmland topsoil samples (0-20 cm) was collected from the Jintan District, Changzhou City, Jiangsu Province, for the analysis of soil total phosphorus contents. Apart from soil total phosphorus, we introduced other environmental factors, such as the distance from receiving water body, phosphate fertilizer application rate, and surface runoff potential, into the assessment of the loss risk of soil phosphorus. Furthermore, the prediction of risk indices based on limited samples inevitably has certain spatial uncertainty. Here, a rapid phosphorus index (RPI) assessment model was firstly constructed based on the multiple environmental factors introduced above. And then, the robust geostatistical methods were applied for identifying possible spatial outliers of soil phosphorus data, and the sequential Gaussian simulation (SGS) model with the dataset being removed the spatial outliers was used to simulate the possible spatial distribution pattern of soil total phosphorus. Finally, multiple equiprobable realizations of soil total phosphorus and related environmental factors were input into the RPI model to evaluate the risk of soil phosphorus loss and represent associated spatial uncertainty. The results showed that the high-risk areas of soil phosphorus loss were mainly located in the eastern and middle parts of the Jintan District, which were similar with the high-value areas of soil total phosphorus. The dryland soil total phosphorus located in the mid-west of the study area was relatively low, while the loss risk of soil phosphorus was still at a high level. Such spatial distribution patterns of soil phosphorus loss risk were mainly caused by agricultural inputs. Moreover, the environment factors (i.e., the distance from receiving water body, surface runoff potential, etc.) in these areas were also favorable for soil phosphorus loss. The areas with high level risk of soil phosphorus loss represented zonal distribution along the rivers in the study area. The highest risk area with RPI value exceeding 1.06 made up 24.94% of the study area, whereas the high-risk area (RPI value from 0.93 to 1.05) was relatively larger, accounting for an area percentage of 40.94%. Overall, the risk of soil phosphorus loss was high across the study area. In the process of the uncertainty assessment, the critical threshold value was set to be 1.06 for identifying the highest risk area. When the critical probabilities were set to be 0.50, 0.75, 0.85, and 0.95, respectively, the area exceeding the critical threshold value accounted for 16.71%, 5.74%, 2.84%, and 1.04% of the total area of Jintan District. The critical probability of rapid phosphorus index value exceeding the threshold value was greater than 85% in the north area, and around the Taohu Lake. We introduced environmental factors (the distance from receiving water body, phosphate fertilizer application rate, surface runoff potential) to assess the soil phosphorus loss, which can assess the spatial uncertainty of soil phosphorus loss at a county scale quickly. The knowledge of spatial uncertainty is helpful for the decision maker to delimit the critical source areas and regulate the regional soil phosphorus loss.