Abstract: Researches on the variation law of reservoir parameters of both high porosity high permeability and medium porosity medium permeability reservoirs during water flooding in Dagang oilfield indicate that after long time waterflooding reservoirs show regular changes.In high porosity and high permeability re-servoir,contents of clay mineral and cement decrease,dissolution pores increase,pore-throat enlarges and reservoir physical properties are improved.Whereas most of medium porosity and medium permeability reserviors show the trend that pore-throat reduces and reservoir physical properties become poor.The two types of reserviors bear similar trends that the microscopic and macroscopic heterogeneity is enhanced,the wettability changes into strong water-wetting and the properties of crude oil become poor.This research is significant in later development of high water cut reservoir.