<p>A novel composite lap-joint repair method, the mortise-and-tenon lap (MTL) repair, is proposed as an alternative to the conventional stepped lap (SL) repair. To address the issues of stress concentration at interfaces and insufficient load-bearing efficiency in adhesive bonding repair of carbon fibrous composite laminates, the effects of the mortise-and-tenon joint on the repair performance of composites were investigated using finite element analysis and experimental validation. A three-dimensional progressive damage model was implemented in ABAQUS/Explicit via a user defined subroutine (VUMAT) to analyze interfacial debonding between the patch and the substrate in the repaired specimen during three-point bending tests, which revealed differences in damage mechanisms between mortise-and-tenon lap repairs and conventional stepped lap repairs. Results showed that the mortise-and-tenon joint reconstructed the load transfer path, distributed stresses through the tenon region, and exhibited synergistic effects between the tenon and the adhesive interface, thereby effectively retarding damage propagation and mitigating interfacial stress concentration. Additionally, parameter optimization by adjusting patch size (Rply-1 and Rply-2) indicated that repair performance was governed by the synergistic interaction between the mortise-and-tenon joint configuration and the adhesive bonding area. The optimal configuration (MTL-40-28-36) achieved peak load and failure displacement improvements of 28.6% and 11.3%, respectively, compared to the stepped lap joint with the same patch combination.</p> Graphical Abstract <p></p>