In order to reveal the law of phosphorus loss on a slope of karst valley under natural rainfall conditions, the migration process of different forms of phosphorus through the two transport pathways (slope runoff and erosion sediment) was analyzed, and the influence of rainfall conditions and land use patterns on phosphorus loss was emphatically discussed. Three standard runoff plots (20 m×5 m) of cultivated land, forest land, and orchard land were set up in Longfeng trough valley to monitor the phosphorus loss for one year. The results showed that: (1) Phosphorus loss was strongly dependent on the process of runoff and sediment generation. (2) In general, the total loss of phosphorus decreased in the order: cultivated land > orchard land > forest land. The total loss of phosphorus in cultivated land was 0.17 kg/hm², which was about 1.76 times that of orchard land and 4.68 times that of forest land. (3) The change of phosphorus concentration in slope runoff was controlled by the rainfall intensity. The significant form of phosphorus loss in slope runoff was dissolved phosphorus (TDP) at the beginning of the rainy season, while in other rainfall runoff events particulate phosphorus (PP) was the primary form which accounted for 63.92%~96.97% of the total amount of phosphorus runoff loss. (4) The change of phosphorus concentration in sediment was less affected by the rainfall intensity, and the sediment had the enrichment effect of phosphorus, there was no obvious correlation between the enrichment ratio of total phosphorus (ER_STP) and rainfall intensity.