Steel bar corrosion monitoring is a very important part of structure health monitoring and full-life design in civil engineering. Many electrochemical methods have been developed in the last decades for corrosion research, but for field corrosion, pulse perturbation is the fast method. The corrosion of Q235 carbon steel in cement mortar has been researched by potentiostatic pulse method. The solution resistance Rs, the polarization resistance Rp, capacity of double layer and time constant have got by cure fitting. The amplitude of voltage perturbation affected the accurate of result little. The polarization resistance decreases with increasing temperature.