Based on geological interpretation on the 2-D seismic profile as well as outcrops, the paper presents the idea that the basement fault is an important structural boundary, on both sides of which the Mesozoic and Paleozoic strata vary greatly in the sequence and thickness. And the western boundary of Qinzhou-Fangchenggan depression from Hercynian to Indo-Chinese epoch is not Lingshan fault but basement fault. The area of the depression is much bigger than that of the structural zone between Bobo-Luoding-Guangning fault and Lingshan fault. Large area of the Shiwandashan basin is covered by the basement fault, which is an important and potential petroleum exploration area for further working.