The history of oil reservoir waterflooding development in Shahejie formation of QK18-1 oilfield was nearly 20 years. At present，more than 75% of the oil wells were lack of injection，and the economic efficiency of oilfield was low. In order to define the mechanism of reservoir damage and establish an effective method to improve the injectivity of oilfield injection wells，the micro geological features between exploration well and development well reservoir cores were compared and the reason of injection difficulty in this oilfield was analyzed through static compatibility test，water sensitivity experiment，water quality survey，combining the photos of casting thin section and electron microscope. The results showed that the key factor of oil reservoir damage was the formation of CaCO₃ with 10—30 μm particle size as a result of the incompatibility between mixed waste water and formation water. The CaCO₃ fouling in the reservoir was a rhombohedral body with good euhedral degree. The crystal grain distributed in the pore and throat，which resulted in the decrease of reservoir physical property. The salinity of mixed waste water was low，and the re-injection led to water sensitive damage. The excess content of suspended matter in water treatment system was also an important factor which influenced the water injection effect. Aiming at the damage problem of reservoir during oilfield water flooding，precaution and suggestions were put forward to improve the water intake capacity of water injection well and ensure the balance between injection and production of oilfield.