In this study, the long-term operational performance of an on-site NF facility at a full-scale oil-field wastewater desalination plant was monitored. The NF facility with poor permeability due to membrane fouling enables efficient multivalent salt removal (rejections of Mg2+, Ca2+, Fe3+, and Al3+ were approximately 100%). Moreover, a comparison of the cleaning efficiencies of two on-site cleaning modes indicated that PL-007 cleaning helped to improve the effectiveness of subsequent acid cleaning in the removal of inorganic foulants. Furthermore, a spiral-wound NF membrane module harvested from the plant was unfolded and autopsied. The results showed that both anionic polyacrylamide (APAM) and crude oil were identified as the predominant organic matter on the membrane surface and collectively accounted for a substantial fraction (86.3%) in terms of dry weight. Additionally, dissolved organics with a high molecular weight were prone to accumulation on the membrane surface. Multivalent elements, including Mg, Ca, Al, Fe, and Si, were the primary inorganic species in the fouling layer. Among the inorganic elements, Si occupied a high proportion and existed in the form of SiO2 in the fouling layer. According to the autopsy results, organic fouling combined with inorganics was responsible for the decline in the flux.