Abstract: The Mega Basin is a typical intermountain fault basin constained by the faulting around the basin. The study of the Mega ancient lake shows that the formation, development and collapse of the lake are closely related to those of the Mega Basin. The faulting, northern and southern differential uplifting, palaeoclimatic changes and fluvial erosion from the Holocene onwards resulted in the evolution of the basin according to the evolutionary path of river-swamp-river-swamp-lake-swamp-lake-river. The Mega ancient lake has a total thickness of sediments of 20.5 m and an average depositional rate of more than 2.81 mm/a, which are much higher than those of the middle Pleistocene Lanong Lake in the same area. This implies that for the Mega Basin, the subsidence rates during the Holocene are much higher than those during the middle Pleistocene.