The water injection development mode which blended clean water and sewage during reinjection process was widely used in early exploitation stage of offshore oilfield. The clean water and the sewage were produced from Guantao formation and Minghua Town formation in BZ28-2 south oilfield. The salinity of that was 8514.7 and 6605.7 mg/L, and the water type of that was CaCl₂ and NaHCO₃, respectively. Taking BZ28-2 south oilfield as an example, we utilized parameters including formation mineral sensitivity analysis, sensitivity experiments, compatibility experiments of sewage and clean water, indoor core flooding and platforms water quality study to comprehensively analyze formation damage mechanism of the oilfield during water injection; and aiming at the characteristics pertaining to offshore oilfield water injection, we established a set of evaluation method with reference to formation damage mechanism associated with water flooding. Studies showed that the water injection effect of BZ28-2 south oilfield was primarily affected by higher intensity of water injection that caused velocity sensitivity damage. The results from static compatibility evaluation showed that the total amount of scale in regard to clean water was 27.0—70.5 mg/L at 80℃. The total amount of scale in average decreased from 70.5 mg/L to 18.3 mg/L when the temperature decreased from 80℃ to 60℃, which indicated that the clean water had a better scaling ability. When clean water blended with formation water with different volume fraction, the content of suspension scale and settlement scale as well as total amount of scale increased first and then decreased with increasing volume faction of formation water, and the peaks occured where volume fraction was 1:1. The dominant component of suspension scale and settlement scale was CaCO₃. It could be discovered from the results referred to dynamic compatibility evaluation that the damage rate of permeability caused by clean water to the core was 41.11%—89.36%. The clean water injected into formation was incompatible with formation fluid, which made the flow channels be blocked by calcium scaling and consequently the reservoir had difficulty in water injection. Meanwhile, the key factor that the current water injection hadn’t reached requirements was attributed to the platform shorter sewage treatment time as well as larger processing capacity, which had the oil content failed to reach the standard. Specific measures proposed to prevent formation damage, as a result, the absorbing capacity of water injection wells improved and the balance of injection and production in oilfield was ensured.