By using the improved trace (50 μg) Sr isotope analytical method the87Sr/86Sr ratios of Xiaodukou foraminifera fossils were measured, giving a range of 0.71105–0.71274, apparently higher than the value of contemporaneous sea water (0.709087 – 0.709147) and also slightly higher than the average value of modern Yellow River (0.7111), demonstrating that the contemporaneous environment where Xiaodukou foraminifera inhabited was an inland lake. Detailed analyses of ΔSr values showed that there occurred an event responsible for environmental changes in the ancient Nihewan Lake area during the time (about 1.0 Ma ago) when Xiaodukou foraminifera appeared. Because of strong evaporation the salinity of the lake would increase and a regional salt-water or brackish-water lacustrine environment would be produced, thus providing a suitable and inhabitable environment for foramini ferae. It is concluded that Xiaodukou foraminiferal fossil assemblages belong to non-marine foraminiferal species.