Based on the gravity field characteristics along Algan-Laomangya and Golmud Ejin Banner geoscience Transections as well as the relevant geological data, the authors have divided the study area into five second order tectonic units with eleven relatively large faults, calculated and analysed Airy isostatic anomalies and Moho depth. It is inferred that the Central Qilian fault in the Altun fold system is the northern boundary of Qinghai Tibet plateau.