Abstract: This research investigated the reservoir damage of naturally fractured oil reservoirs caused by suspended solids and oil in injected water via laboratory core flooding tests. The results showed that the median diameter of suspended solid particles and oil content are the main factors controlling reservoir damage. With the permeability damage upper limit of 20%, the injected water quality requirements are that the median diameter and content of suspended solids should be no more than 10 μm and 10 mg/L respectively, and the oil content should be no more than 10 mg/L, for fracture widths ≤50 μm; the median diameter and content of suspended solids, no more than 20 μm and 10 mg/L respectively, and the oil content no more than 20 mg/L, for fracture widths of 50–100 μm; the median diameter and content of suspended solids no more than 50 μm and 50 mg/L respectively, and the oil content no more than 40 mg/L, for fracture widths of 100–200 μm; the median diameter and content of suspended solids no more than 60 μm and 80 mg/L, respectively, and the oil content no more than 50 mg/L. The findings of this research laid down the theoretical basis for high-efficiency water flooding of fractured oil reservoirs.