The dolomite of the Sha 1 lower submember in Qikou Sag was developed under the background of lake expansion. Through the microscopic identification, scanning electron microscopy analysis and geochemical analysis and testing on the dolomite samples in the study area, some fossils indicating marine facies were discovered such as glauconite, collophanite and coccolith. It was also found that the lake basin was once effected by the sea water during the same period through the analysis of Sr/Ba ratio, V/Ni ratio and 87Sr/86Sr ratio, which demonstrates that the transgression activities not only provide part of Mg2+ for the formation of offshore lake basin dolomite, but more importantly, change the nature of the lake basin water so as to promote the dolomitization. The ancient landscape with alternating “depression” and “uplifting” in the study area, and good correlation of carbon and oxygen isotope by its linear distribution suggest the lake basin had a poor circulation and good closure during the Sha1 lower sub member period, which led to sustained salinization environment of the basin caused by the transgression, ensuring a favorable environment for the development of the dolomite.