Abstract: Many low-resistivity oil reservoirs have been found in the medium-deep buried strata of the Bohai oil field recently. Most of them have high production after DST. In some of them, the reserve of low-resistivity reservoir may be above sixty percent. It is indeed an urgent need to find out the causes and evaluation method for this kind of reservoirs. Upon the evaluation experiences from other fields, we studied in this paper the origin of the low-resistivity oil reservoir in the medium-deep strata, and the methods to identify them from others. The results show that complex pore structure, high salinity water invasion, and change in water salinity are the main causes for the formation of low-resistivity oil reservoirs. The methods, which may effectively identify the low-resistivity oil reservoir, are also discussed, such as the combination of conventional well logging technique, mud logging technique and some new well logging techniques like NMR logging and array acoustic logging. The combination is proven effective.