In order to provide references for the safe use in engineering applications, the corrosion behavior and post-corrosion mechanical properties of HRB400 reinforcement steel, exposed to a 3.5% NaCl solution in two distinct environments-bare and embedded in concrete were investigated through immersion corrosion test. For HRB400 reinforcement steel embedded in concrete, two approaches were employed: the fabrication of reinforced concrete and the simulation of concrete pore fluid. Initially, HRB400 reinforcement steel, both in bare form and embedded in concrete, was immersed in a 3.5% (mass fraction, the same below) NaCl solution. And soaking the HRB400 reinforcement steel in a saturated calcium hydroxide solution (with addition of 3.5% NaCl solution). The weight loss rate and corrosion rate of the HRB400 reinforcement steel under different corrosion times were calculated separately. Subsequently, the mechanical properties of the corroded HRB400 reinforcement steel were assessed using MTS testing machine. Results showed that as the immersion corrosion time prolonged, the average corrosion rate of HRB400 reinforcement steel across the three environments gradually decreased, while the rate of weight loss increased. Additionally, there was a decline in yield strength, tensile strength, and elongation rate. In the 3.5%NaCl solution, the elongation of HBR400 reinforcement steel in bare form and embedded in concrete decreased with the extension of soak time. However, in the saturated calcium hydroxide solutionl (with addition of 3.5%NaCl solution), the elongation showed a trend of mcreasing and decreasing.