The foundation pit support scheme of diaphragm with inner support was made according to the characteristics of Yongtai station for Guargzhou metro line, where the deep foundation pit was long and with many underground karst caves and bad geological conditions. The effect of the major factors to the die- phragm deformation, including the thickness of diaphragm，the stiffness of the inner support and the pre- axial force, were analyzed by use of the ANSYS. The thickness of diaplmagm and the stiffness of inner sup- port are confirmed according to the relationship curves of the two versus the maximum displacement. And then, the pre-axial force of the steel support is adjusted to make the maximum displacement of diaphragm acceptable. The monitoring results show that the displacements of the diaphragm and the settlement are about 30mm, therefore the supporting scheme of the deep excavation is reasonable and can serve as a refer once for similar projects.