Frost resistance of full scale (150mm×300mm×2700mm) reinforced concrete(RC) beams were studied by using air freezing and air thawing regime. The influences of freeze thaw cycles on the relative dynamic elastic modulus, damage layer thickness, compressive strength and mass of RC beams were investigated. And the influences of immersion time and reinforcing steel on ultrasonic test results were studied. Meanwhile, the influence of freeze thaw cycles on the ultimate bearing capacity of RC beams was clarified and the ultimate bearing capacity deterioration model of RC beams under freeze thaw cycles was established. The results show that the relative dynamic elastic modulus of RC beams gradually decreases to 054 after 150 times of freeze thaw cycles, the damage layer thickness of RC beams increases under freeze thaw cycles, while compressive strength of RC beams decreases under freeze thaw cycles. The mass of RC beams has a slightly raise of 09% after 125 times of freeze thaw cycles, and the ultimate bearing capacity decreases to 867% that of the control RC beam after 150 times of freeze thaw cycles.