Although many velocity and electrical models have been proposed in the Longmenshan fault zone (LFZ) and its neighboring areas, the deep structure of seismic gap and the geodynamics of two different earthquakes remain uncertain. Based on aeromagnetic and gravity data, the Sichuan basin shows two NE-trending banded and strong positive magnetic anomalies and high Bouguer gravity anomalies. The banded magnetic anomalies represent the Neoproterozoic magmatic events in the center of Sichuan basin, rather than the rigid Neoarchean and Paleoproterozoic crystalline basement. The Songpan-Ganzi fold belt (SGFB) is weak positive magnetic and low Bouguer gravity anomalies. The LFZ is the boundary of two anomaly areas but similar to the feature of Sichuan basin. Three models are created by 2D magnetic and gravity forward modeling and provide more reliable and integrated geophysical interpretation for the deep structure of earthquake epicenter and seismic gap. The models reveal that the crust of Sichuan basin consists of double layer magnetic basement. More importantly, the basement subducted to about 33km west of the Wenchuan-Maoxian fault with low dip angle beneath the middle segment of the LFZ, whereas the distance decrease to about 17 and 19 km under the south segment. So, the crust of Sichuan basin beneath the middle segment extends further than the one beneath the south segment with the seismic gap as transition zone. Therefore, we propose the irregular shape of basement in western margin of Sichuan basin maybe the main reason for the different focal mechanism and geodynamic of Wenchuan and Lushan earthquakes.