The microscopic theory of superconducting (SC) state in the SU(2) slave-boson model is developed. We show how the pseudogap and Fermi surface (FS) segments in the normal state develop into a d-wave gap in the superconducting state. Even though the superfluid density is of order x (the doping concentration), the physical properties of the low lying quasiparticles are found to resemble those in BCS theory. Thus the microscopic theory lay the foundation for our earlier phenomenological discussion of the unusual SC properties in the underdoped cuprates.