Molecular dynamics (MD) simulations were used to investigate the load-displacement curve, pressure distribution and phase transformation during the adhesive contact process between a silica spherical tip and an atomically smooth silicon surface. The jump-to-contact point and the maximum attractive force point during the loading stage were observed, which were caused by the van der Waals interaction and bonding formation between the tip atoms and the substrate atoms, respectively. During the unloading process, the necking separation between the tip and substrate occurs at the maximum attractive force point. At the jump-to-contact point, the atoms in the contact region are under tensional forces. After the jump-to-contact process, the atoms in the contact region are under compressive forces while those around the contact region are under tensional forces. The pressure of atoms in the contact region is increased with the indentation depth. During the unloading process, the atoms in the contact region returns to the state under tensional forces at the maximum attractive force point. The analyses of the phase change process shows the generation of high pressure phase in the contact region during the loading process. During the necking separation period in the unloading process, the atomic number with the coordination number of 0 is increased rapidly, and more atoms adhered onto the tip to be taken away.