  The surface structure of the SrTiO$_3$(110) polar surface is studied by
scanning tunneling microscopy and X-ray photoelectron spectroscopy. Monophased
reconstructions in (5$\times$1), (4$\times$1), (2$\times$8), and (6$\times$8)
are obtained, respectively, and the evolution between these phases can be tuned
reversibly by adjusting the Ar$^{+}$ sputtering dose or the amount of Sr/Ti
evaporation. Upon annealing, the surface reaches the thermodynamic equilibrium
that is determined by the surface metal concentration. The different electronic
structures and absorption behaviors of the surface with different
reconstructions are investigated.
