In this paper, a damaged R.C. frame with special-shaped columns has been strengthened for earthquake resistance. The strengthening method combines wrapping steel outside and stick steel. The experimental research on seismic behavior of the strengthened frame was performed. Through the quasi-static test, the load-displacement hysteretic curves under horizontal low-cycle loading are given, and the failure modes of the strengthened frame structure are summarized. This paper gives the analysis of its bearing capacity, stiffness deterioration, energy dissipation capability and ductility property. After comparing the strengthened frame with that before damaged, we draw a conclusion that the method of seismic strengthening in this paper can effectually improve the carrying capacity of damaged R.C. frame with special-shaped columns, and make the strengthened frame have better ductility.