In arid regions, the coexistence of soil salinity and organic contaminants presents a formidable challenge for remediation. In this study, a composite photocatalyst of MIL-88B(Fe) and biochar (MIL-88B/BC10) was prepared via a facile one-step in situ growth method. The strong interfacial interactions within the composite enabled a unique synergistic remediation effect, effectively removing refractory polyaromatic hydrocarbon while simultaneously improving salinized soil quality. Under salinized soil conditions, this synergistic effect achieved a Phenanthrene (PHE) removal efficiency of 83.93%. This high performance was attributed to the incorporation of BC, which enhanced the adsorption and activation of PHE at the active sites, while acting as a support to effectively mitigate the aggregation of MIL-88B(Fe). The formation of Fe-O-C bonds at the MIL-88B/BC10 interface optimized the band structure, facilitating the separation and migration of photogenerated carriers, and thereby improving the photocatalytic performance. Through DFT calculations, trapping experiments, and GCMS, the adsorption and degradation mechanisms and pathways were elucidated. Electrons flowed from MIL88B(Fe) toward BC, generating a strong built-in electric field at the interface. The relative contribution of reactive species followed the order: center dot O-2- (81.6%)> center dot OH (66.5%)> SO4 center dot- (39.3%). Under the action of reactive radicals, PHE was progressively decomposed into long-chain acids, long-chain esters, and aliphatic compounds, which were eventually mineralized into CO2 and H2O. Furthermore, the salinized soil was effectively remediated, as evidenced by the reduction in both salt ion content and pH. This study provides valuable insights into the design of eco-friendly photocatalysts and the efficient degradation of organic pollutants in salinized soils.