In recent years, with the exploration of the ocean, many marine engineering projects have emerged, followed by various issues related to the durability design of marine engineering structures. Among them, the corrosion of rebar in reinforced concrete structures caused by chlorine salts in seawater is one of the most serious problems. A series of issues related to rebar corrosion in concrete, including the difficulty in predicting the rebar corrosion initiation, the degree classification of general corrosion to severe corrosion, etc., are significant problems related to the durability design of reinforced concrete structures in marine engineering. Two important reasons are the inaccuracy and disunity of the determination methods for steel bar rust degree leading to the large dispersion of chloride concentration threshold used to predict these critical thresholds and the unclear mechanism of the effect of chloride concentration on the localized corrosion kinetic process of rebar. In order to better understand the localized corrosion of rebar in concrete, this work introduced a localized corrosion theoretical framework and improved localized corrosion testing techniques to explain the effect of chloride concentration on the localized corrosion kinetic process. Determined with the theoretical parameters of localized corrosion such as repassivation potential, critical corrosion temperature, cyclic potentiodynamic polarization curves, etc., which related to the chloride concentration threshold directly. Based on the theoretical analysis and experimental results, the essential law of localized corrosion in concrete will be discussed and a more convincing and appropriate method to judge the rebar corrosion initiation will be suggested. The recent concept of a critical threshold of chloride concentration for rebar corrosion initiation will also be reconsidered, to improve the accuracy of the prediction of initial corrosion time and classification of corrosion degree of steel bars. These results are expected to provide theoretical guidance for the durability design and improvement methods of reinforced concrete structures.