To evaluate improvements in the flexural capacity and deformation behavior of shield tunnel segments reinforced with prefabricated composite materials, a repair method that combines externally bonded basalt fiber unidirectional fabric with steel pipes is utilized. The segments are internally filled with prestressed grouting material. This reinforcement technique is implemented on shield tunnel segments from a metro project in Beijing, China. Four-point flexural tests are conducted to systematically compare the behavior of shield tunnel segments before and after reinforcement. The results elucidates the impact of the composite strengthening approach on failure mechanisms, flexural capacity, and strain distribution profiles. Based on the experimental findings and strain distribution data, an ultimate failure criterion is proposed, and the reinforcement mechanism is interpreted through a comprehensive analysis of the structural response. The experimental results indicate that: (1) Prior to reinforcement, the segment exhibits vertical through-cracks dominated by circumferential fractures at the bottom during loading, whereas after reinforcement, the failure mode shifts significantly to diagonal cracking initiated from the mid-span toward the loading points. (2) The ultimate bearing capacity of the segments strengthened with prefabricated composite profiles increases from 523 to 927 kN by 77.2% through the triple mechanism of external restraint, interface bonding, and internal prestressed grouting. (3) The strain curves at the bottom of the reinforced segments show adeviation of less than 5% from the unreinforced ones in the elastic stage, indicating strong deformation compatibility between the composite material and the concrete, and excellent synergistic load-bearing performance.