Abstract: Lower crust flow is often used as the key to explain the phenomenon of depth-dependent stretching. However, quantitative studies of the flow and its effects on crustal deformation are rare in lietrature. In this paper, the lithospheric extensional deformation of the Baiyun Sag is selected as a case of study using the Pratt-Hayford isostatic model. Assuming that the original crustal thickness (i.e. equilibrium depth) of the Baiyun Sag is 32 km, the original crustal thickness estimated from the observed thicknesses of the basement and sedimentary cover, and the water depth will be larger than this figure and within the range of 32.599-33.774 km. It becomes thicker from the continental shelf to the ocean basin due to the increase in mantle material upwelling caused by the lower crustal loss. According to the calculation, the lower crustal loss increases from the ocean towards the shelf direction. The loss of lower-crustal flow results in the decrease in the whole crustal extension factor in the Baiyun Sag up to the value of the ocean basin.