[Purposes] The paper aims to address damage and deterioration of segments due to the aging of shield tunnels, improve their service performance, and extend their service life. [Methods] An interval tunnel at urban rail transit was selected as the research object. A method for reinforcing shield tunnel lining segments by grouted channel steel was proposed, and full-scale tests were conducted to investigate the bending load failure mechanism of segments and the performance improvement of damaged segments after reinforcement. Analysis was made on the variations of the failure characteristics, crack development, and parameters (such as strain) of concrete and the steel plate before and after reinforcement. [Findings] The grouted channel steel reinforcement method can significantly enhance the loading capacity of damaged segments, with the ultimate load increasing by approximately 33.0%. This method can effectively prolong the elastic and plastic phases of the damaged segments and limit the growth of crack widths. The failure characteristics of the segments before and after reinforcement are basically the same. First, through cracks appear at the loading point of the inner arc surface, and with the increase in the applied load, the cracks gradually spread on the inner side of the loading point and develop toward the restrained bearing end. The final failure mode of the reinforced segments is obvious deformation of the bolt at the flange at the channel steel end and flange tearing, which leads to abrupt increment of crack width on the segments. [Conclusions] In summary, grouted channel steel reinforcement of shield tunnel segments is feasible, and special attention should be paid to the strength of reinforced channel steel and the effective connection between the reinforced channel steel and the segments.