Due to the complexity of its geological structure and geological evolution, Longmen Mountain has always been the focus of research. The middle part of Longmen Mountains, which has large topographic gradient changes and the most complex magmatic metamorphism, has become one of the ideal areas to reveal the geological structure of Longmen Mountains. At present, there is no unified understanding of several stages of structural activities in the middle part of Longmen Mountains during the Indosinian period. In this paper, we make full use of drilling and seismic data and joint well and seismic demarcation to identify and classify the Indosinian unconformity in Mianzhu area in the piedmont zone of middle section of Longmen Mountains. According to the plane projection of the cut overlap points of the unconformity, we outline the distribution range of various types of unconformity in different periods, construct the chronostratigraphic framework in Mianzhu area, and establish the unconformity evolution model map. The results show that there are four consecutive stratigraphic faults in the early Indosinian stage, 5Myr in the middle Indosinian stage I, 5Myr in the Middle Indosinian stage II, and 28Myr in the late Indosinian stage. Four sets of unconformity were identified from the bottom up during the Indosinian period in the study area: unconformity between Leikoupo Formation and Majantang Formation, unconformity between Xiaotangzi Formation and Member 2 of Xujiahe Formation, unconformity between Member 3 and Member 4 of Xujiahe Formation, unconformity between Member 5 of Xujiahe Formation and Baitianba Formation. In general, the unconformity spread from NE to SW along the Indosinian ancient nappe, which was wide in NE and narrow in SW. From NW to SE, the unconformity changed from angle unconformity to parallel unconformity. The middle part of Longmen Mountains is characterized by multi-stage evolution and progressive deformation from north to south, among which the early and late Indosinian phases are the key structural changes affecting Longmen Mountains to Sichuan Basin. The determination of the spatial and temporal distribution of the Indosinian unconformity in the piedmont zone of middle part of Longmen Mountain and the establishment of the chronostratigraphic framework provide reference for the quantitative study of the Indosinian tectonic belt of Longmen Mountain, and provide a basis for the order of the Indosinian tectonic activity.