The properties and。distribution characferistics of fluids in complicated fault-block oil and gasfields have been studies on the basis of geological data from the Dongxin and Yonganzhen oil—gasfields．The study results show that the properties of fluid in fault—block oil-gas field are highlycomplicated and controlled by the burial depths of the reservoirs；with increasing buried depth，oil become light，gas becomes heavy and the salinity of formation water increases．It is pointedthat secondary change is the main factor determining the changes of oil properties．