Abstract: Zizhen sag, a small asymmetrical sag in Jiyang depression, is explored to a low degree. The paper describes the structural characteristics of Zizhen sag from three aspects as follows:formation evolution, the spatio-temporal differences of margin fault activity and the transference of subsidence center. Besides, four formation evolution phases of Zizhen sag are divided:three rifting stages in Kongdian-Dongyin formation and one subsiding stage in Neogene. The spatio-temporal differences of the Jishan fault activity lead to the transference of subsidence and form two secondary sag in the east and west respectively.