Adsorption behavior and electronic structure of tin-phthalocyanine (SnPc) on Ag(111) sur-face with Sn-up and Sn-down conformations are investigated using first-principles calcula-tions. Two predicted adsorption configurations agree well with the experimentally deter-mined structures. SnPc molecule energetically prefers to adsorb on Ag(111) surface with Sn-down conformation. The energy required to move the central Sn atom through the frame of a phthalocyanine molecule, switching from the Sn-up to Sn-down conformation, is about 1.68 eV. The simulated scanning tunneling microscopy images reproduce the main features of experimental observations. Moreover, the experimentally proposed hole attachment mech-anism is verified based on the calculated density of states of SnPc on Ag(111) with three different adsorption configurations.