Through experiments of corroded steel bar specimens under cycling tensile load, the effect of corrosion on mechanical property was analyzed. It's revealed that the mechanical properties and deformability decrease gradually, the yielding plateau gradually shortens or even disappears, energy-dissipation capacity decreases, which makes the anti-seismic performance of structure degenerate, even sudden collapse can happen more easily in earthquakes. Based on the test results, the constitutive model for stress-strain relationship of corroded steel bars was established, which can be used as a reference to the appraisal for residual bearing capacity, durability and seismic behavior of reinforced concrete structure for the future.