Abstract: In water flooding sandstone reservoir，injected water has large impact on crude oil properties.The density，viscosity，wax content and freezing point of crude oil gradually increase along with the time of water flooding development.The change of crude oil viscosity shows the feature in stages that the viscosity increases sharply before the stage of 20% water cut，and increases smoothly in the stage of 20% to 85% water cut，then increases rapidly after the stage of 90% water cut.The change of crude oil properties is different in different areas or layers.The relationship among crude oil properties also changes before or after water breakthrough.After water breakthrough，the viscosity of oil with the same density is larger than before，and the freezing point of oil with the same density or viscosity becomes higher than before.This study on the change mechanism of crude oil properties shows that the change of reservoir temperature，separation of light component from crude oil，degassing and injected water quality are the primary causes of the change of crude oil properties.

Key words: water flooding development, crude oil properties, water cut, injected water quality, reservoir temperature