Although it is well accepted that the Mianyang-Changning intracratonic sag plays an important role in petroleum accumulation, its spatio-temporal distribution and formation mechanism are still controversy. In this case, this study carries out systematic stratigraphy and lithofacies investigations on the Sinian-Cambrian successions of the Guangyuan-Ningqiang area, at the northeastern margin of the Mianyang-Changning intracratonic sag. The lithofacies indicators and biostratigraphy suggest that the lower Beiwan Member or upper Algal dolomite Member are commonly overlain by Kuanchuanpu Formation, and thus the Gaojiashan and Beiwan members are pervasively missing at Kuanchuanpu, Shujiaba and Caojiaba-Dongshanzi areas. This indicates that the strata missing of the upper Dengying Formation at eastern margin of the Mianyang-Changning intracratonic sag can extend from the Central Sichuan Basin and north slope of Central Sichuan Paleouplift to the Guangyuan-Ningqiang area. The sharp shift from tidal flat-lagoonal microbial dolomite and dolomudstone in the study area to slope-basinal chert at Datan-Yangmu area indicates the control of extensional fault. Similar sharp shift in facies is also present at the southern margin of middle Yangtze platform and eastern margin of the upper Yangtze platform, reflecting an extensional tectonic regime with the formation of fault-bounded carbonate platforms. The width of several kilometres, erosion thickness of hundreds of metres and the depositional break up to 20 Myr suggest that Mianyang-Changning intracratonic sag can be attributed to differential subsidence/uplift rather than karstic morphology. The linear distribution of strata missing area at eastern margin of the Mianyang-Changning intracratonic sag should be resulted from relative uplift at the tilted block. The climax of extensional faulting occurred at the Sinian-Cambrian transition, causing the strata missing of the upper Dengying Formation to the lowermost Kuanchuanpu Formation.