Abstract: Long-term discharge may negatively affect marine ecosystems around submarine sewage outfalls, including pollutant accumulation in sediments and degradation of benthic ecosystems. However, surveys conducted in 2002, 2011, and 2018 on sediment quality, sediment type, and benthic communities in the waters adjacent to the Qianpu outfall in Xiamen revealed that, after 16 years of discharge, the sedimentary environment remained relatively minimally impacted. Specifically, the interannual variations in the average concentrations of organic carbon, sulfides, and petroleum hydrocarbons in the sediments were not significant. The maximum concentrations observed across all periods complied with the Class I Marine Sediment Quality Standard, and their spatial distribution did not show a decreasing trend with increasing distance from the outfall. Sediment grain size showed a dynamic pattern of “coarse-fine-coarse”, but sandy sediments consistently dominated, with no surface hardening or flocculation observed on the surface of sediments. The benthic community remained stable, with species number ranging from 113 to 148, biomass from 15.47 to 105.40 g/m², and density from 149 to 664 ind/m² across the three survey periods. The diversity, evenness, and richness indices were consistently moderate to relatively high, and the dominant species were primarily polychaetes and crustaceans. No strong correlations were identified between sediment quality and benthic ecological indices. This study indicates that the favorable environmental conditions are primarily due to the region’s strong hydrodynamics, ecosystem resilience, and stringent effluent quality requirements. Accordingly, for marine outfalls, it is recommended to standardize siting and ensure compliant effluent discharge, implement long-term monitoring of “water quality-sediment-benthic communities”, and establish a post-environmental impact assessment mechanism.