Yitong graben is a strike-slip and extensional basin in Meso-Cenozoic, the eastern China. After seismic profile explaining and well core observing, a large-scale sublacustrine fan complex with an area of 400km2 and thickness of 250m was identified at the bottom of Member Yong1, Paleogene of the graben. As the low-stand fan of three-order sequence Yong1, It was mainly composed of fine to middle-grained sandstone and a little gravel stone which bore many sandstone breccias, mudstone shivers or muddy gravels together with various pene-contemporaneous deformation structures such as convolute bedding, liquefied distortion, ball and pillow structure, microfault and so on, so it was determined to be dominantly subaqueous debris flow deposit due to slump. After underlying Sheling Formation had developed, the research area suffered reverse tectonic extrusion, denudation, and then violent normal-faulting accompanied with volcano breaking forth and earthquake activity. The fan at Member Yong1 was synthetically caused by above tectonic affairs, and it indicated that a new episode of tectonic subsidence occurred and that the garben came into a new evolution stage. Owing to the farraginous lithology composite and very poor reservoir capacity, this sublacustrine fan was not favorite to oil and gas accumulation although its size was very large.