The study and identification of the extensional structures in the pre-Sinian to Cambrian strata confirm the extensional tectonic setting during the late Proterozoic-eaHy Palaeozoic in Sichuan Basin. The paper proposes the view of the existence of the Early Cambrian Mianyang-Lezhi- Longchang-Changning intracratonic sag by interpreting the drilling well information and seismic profiles. In the section, the Early Cambrian intracratonic sag possesses the general characteristics of half-graben. Its boundaries are steep on the east side and relatively gentle on the west side. The east steep boundary is more obvious in the middle segment of the sag, and weak in the north and south segments. On the plane, the distribution of the thickness of Lower Cambrian Canglangpu, Qiongzhusi and Maidiping Formations shows that the sag is wide in the north and south segments, and narrow in the middle segment. The determination of the early Cambrian intracratonic sag is of great importance to the petroleum exploration in Sichuan Basin.