The oxygen isotopic composition of ostracod shells has been used as a useful indicator in plaeolimnological research. But studies on the oxygen isotopic fractionation of ostracod shells and its environmental significance were still not enough. In this study, surface sediments and co-existing water were systematically collected at different sites in Lake Qinghai. The oxygen isotopic composition of lake water and ostracod shells of Limnocythere inopinata Bird selected from the surface sediments were measured to test the oxygen isotopic fractionation of ostracod shells and evaluate its environmental significance in Lake Qinghai. Our results show that the oxygen isotopic composition of L. inopinata is mainly controlled by the water isotope and the oxygen isotopic fractionation between ostracod shells and water is not only affected by temperature but also maybe by water salinity. The oxygen isotopic fractionation factor of ostracod shells reduces with increasing water salinity, eliminating the effect of temperatures. However, the oxygen isotopic compositions of both ostracod shells and water become enriched with increasing salinity in Lake Qinghai. The oxygen isotopic composition of L. inopinata may be used to indicate the variability of salinity in Lake Qinghai, and further works need to be done in future.