Abstract: Rotation is a relatively fixed crop combination pattern that forms in various places over a long period. It is also the basic unit for the calculation of farmland nutrient balance. The accumulation of nutrient surplus or deficiency over the years directly affects the environment. Among the three essential nutrients for plants, the most important for the environment are phosphorus and nitrogen. Compared with nitrogen, the phosphorus cycle is relatively stable, has no gas loss, and reveals a high correlation between apparent nutrient balance and environmental pollution of phosphorus, and the calculation for which is convenient. Therefore, phosphorus was selected as the indicator for environment risk in this study. In order to establish a simple and effective method to detect early warnings of environmental effects, starting with the screening of key elements suitable for early warning and key links to phosphorus loss in farmland, this article proposed an early warning indicator system based on farmland plots suitable for small-holder farmers in China. The study also performed trial calculations with survey data from 38 plots, including five typical rotation systems in North China. The early warning system included three evaluation indicators-annual phosphorus balance, plough layer soil texture, and annual phosphate fertilizer operation. A total of six early warning levels of phosphorus loss was set in the early warning system-dark green, light green, light yellow, deep yellow, light red, and deep red, in which the red meant high phosphorus loss risk, the green meant low phosphorus loss risk, the yellow meant medium phosphorus loss risk, deep and light mean degree of the level. The characteristics of the early warning system were 1) measuring the entire rotation cycle rather than a single crop; and 2) the phosphorus balance considered the total balance of the anniversary (i.e., the phosphorus input and output balance of the entire rotation cycle) and considered the difference between different crops. Scientific allocation (i.e., phosphorus co-ordination) used relative equilibrium values rather than absolute equilibrium values so that the phosphorus balance between different rotation types could be uniformly quantified and compared. Soil texture was simplified to sand, loam, and clay species. The trial results showed that a low early warning level (dark green) which was rational in application of phosphate fertilizer with reasonable application rate and co-ordination between rotation crops only occupied 10.53% of all plots analyzed, while the high environmental risk (dark red, light red) rotation plots accounted for 57.89%, i.e., more than half of the plots using the current rotation methods had high or extremely high environmental risks. The early warning system was scientific, practical, and simple, and the required data were easy to be obtained. It can be used for farmers to judge the risk of phosphorus loss in their own plots and for national, regional, or local agricultural authorities to reference when adjusting planting structures.