This study investigates the spatiotemporal distribution patterns and key influencing factors of carbon, nitrogen, and phosphorus in sediments of a large surface-flow constructed wetland during long-term remediation of slightly polluted water.We collected sediment samples from 28 sites in Yanlong Lake and analyzed the contents of total organic carbon (TOC), total nitrogen (TN), and total phosphorus (TP). Correlation analyses were performed among these parameters, followed by comprehensive pollution and organic pollution assessments. Results revealed average concentrations of 529 mg/kg TP, 953 mg/kg TN, and 0.85% TOC in the sediments. Strong positive correlations were observed between TN and TP (r=0.729, P<0.01) and between TN and TOC (r=0.251, P<0.05), suggesting shared organic sources for nitrogen and carbon. Key spatial patterns emerged: ① Along the water flow direction, TP and TN contents showed decreasing trends across different zones, with the pretreatment area exhibiting the highest nutrient levels and most severe pollution; ② Vertical distribution displayed surface enrichment (0-30 cm) of nutrients across all zones. Compared to 2017 data, the emergent plant zone demonstrated decreasing nutrient trends along flow direction but higher TP and TOC contents, attributable to accumulated algal debris, particle-bound pollutants, and microbial-mediated nutrient release at the inlet area. Pollution assessment using comprehensive and organic pollution indices indicated generally good sediment quality throughout Yanlong Lake. Nitrogen and phosphorus pollution ranged from moderate in the pretreatment zone to clean/mild in other areas, while organic pollution was mild only in pretreatment and advanced treatment zones but clean elsewhere. These findings provide valuable scientific references for sediment pollution evaluation in large surface-flow constructed wetlands during extended operation.