The Paleogene Madingo Formation in the Lower Congo Basin developeda set of phosphorus- containing mudstone rich in organic matter. The overlying strata is silty mudstone of the Paloukou Formation, and the underlying strata is ordinary mudstone of the Madingo Formation. The petrological characteristics of phosphorus- containing mudstone, ordinary mudstone and silty mudstone are studied using thin sections, scanning electron microscope, energy spectrum and surface scanning. Redox conditions of paleo- ocean during the sedimentary period of the phosphorus- containing mudstone, ordinary mudstone and silty mudstone are analyzed based on the test of primary, trace and rare earth elements.The phosphorus source and the mechanism of phosphorus enrichment during the sedimentary period of the phosphorus- containing mudstoneare evaluated by mathematical statistical methods such as correlation analysis and cluster analysis.The results show that the microflora,mainly foraminifera,arewell developed in the phosphorus- containing mudstone. Phosphate particles mainly exist in the form of colloidal aggregates in phosphorus- containing mudstone. The water body was stratified and oxidized during the sedimentary period of the phosphorus- containing mudstone. Cerium anomaly of the phosphorus- containing mudstone was mainly related to phosphate minerals. The phosphate minerals with Cerium anomaly formed in the surface oxidation water and precipitated in the bottom water. The bottom water was hypoxic- anoxic, resulting in the enrichment of redox sensitive elements such as U and V.Phosphorus sources in the marine sedimentary rocks include terrestrial sources, hydrothermal sources, biological sources, air sources and cosmic sources.It is considered that the biological source is the main source of phosphorus in the phosphorus- containing mudstone in the study area. Organisms play an indirect role in the process of phosphorus enrichment in the phosphorus- containing mudstone, which is mainly affected by biomass and redox conditions.Biology is not only the source of phosphorus, but also changes the redox conditions of the medium to release and precipitate phosphorus from phosphorus- containing compounds.