As a result of more than 20 years'experimental studies, we have gradually understo-od the mechanism and behaviors of groundwater-borne uranium in oil-gas geochemical prospecting, realized its index significance for oil and gas and applied it in oil-pas geochemical prospecting.As oil-field water and deep groundwater lie in a reduction or weak reduction environment, the concentration differentiation effect of uranium is not obvious; in contrast, phreatic water is in an oxidation enviornment and hence exhnoits apparent concentration differentiation effect, which undoubtedly preserves information related to oil and gas. This paper describes characteris-tics of uranium anomalies in phreatic water over the oil and gas accumulation, and deals with properties of the uranium index, i. e. stability, direct perceivability and singleness. With the southern part of Songliao central depression and Dongpu depression as examples, the paper also introduces its practical application.