The quantitative assessment of P contamination in sediments is a challenge due to sediment heterogeneity and the lacking of geochemical background or baseline levels. In this study, a procedure was proposed to determine the average P background level and P geochemical baseline level (GBL) and develop P geochemical baseline functions (GBF) for riverbed sediments of the Liao River Watershed (LRW). The LRW has two river systems - the Liao River System (LRS) and the Daliao River System (DRS). Eighty-eight samples were collected and analyzed for P, Al, Fe, Ca, organic matter, pH, and texture. The results show that Fe can be used as a better particle-size proxy to construct the GBF of P (P(mg/kg)=39.98+166.19×Fe (%), R2=0.835, n=66). The GBL of P was 675mg/kg, while the average background level of P was 355mg/kg. Noting that many large cities are located in the DRS watershed, most of the contaminated sites were located within the DRS and the riverbed sediments were more contaminated by P in the DRS watershed than in the LRS watershed. The geochemical background and baseline information of P are of great importance in managing P levels within the LRW. •A procedure is firstly used to estimate P background and baselines in sediment.•P background content is calculated for the Liao River Watershed sediment.•P geochemical baseline level is elucidated for the LRW sediment.•P geochemical baseline function is constructed for the LRW sediment.•This work confirms Fe can be used as a better proxy for P estimation in sediment.