Objectives: The Mesozoic—Cenozoic transgression of the Southwest Tarim Basin has conventionally been thought that the sea water came from western Alay region and the marine feature of the strata was gradually getting weak from west to east. Recently, on the basis of feraminifera fossils, authors have been found that the direction of the Middle—Late of Late Cretaceous transgression and marine feature in this basin once changed. Studying on this change is of great significance to reconstruct tectonic paleogeography and paleo environment for this basin.Methods: In the South Southwest Tarim Basin, authors have firstly found the Middle—Late Late of Cretaceous feraminifera fossils in two localities. The first locality is the Arcchey section where exist 18 genera and 26 species feraminifera fossils, in which the planktonic type and trochspiral type occupy 15% and 74% of the total amount, respectively. The second site is the drilling core from 6903.93 to 7079.81 meters of well depth in the New 2 well at northern Pishan County, where preserve 12 genera and 39 species benthic feraminifera fossils in which the serial type and lagene type occupy 45% and 50% of the total amount, respectively. According to paleoecological analysis for feraminifera fossils as mentioned above, the paleo environment represent normal shallow sea with deeper water. Nevertheless, the feraminifera fossils are very poor or have not been observed, representing upper intertidal zone or lagoon in the North Southwestern Tarim Basin. Results:The occurrence of feraminifera fossils shows that the Senonian—Maastrichtian normal shallow sea was distributed on leading edge area of the Kunlun Mountains of the South Southwest Tarim Basin. The marine feature of the south is strongerly demonstrated than the north, and direction of transgression once changed. Conclusions:The marine feature of the Senonian—Maastrichtian strata in the south of this basin, i.e., the Kumlun Mountains front region, is stronger than the north, i.e., the Tianshan Mountains front region. Thus, the south—north differentiation occurred, and the northern margin of the sea was located on the edge of this basin. The discovery of feraminifera fossils in the New 2 well and nannofossils in the Maigaity slope has implication in that the distribution scope of the marine strata is wider than the conventionally recognized. During the Senonian—Maastrichtian, the sea water came from the West Kunlun Ocean and invaded from south to north in this basin.