Abstract: This paper investigates the mechanical properties and damage evolution of single lap joints of carbon fiber reinforced polymer (CFRP)/aluminum alloy connected by countersunk titanium alloy blind rivets under multiple parameters. A three-dimensional numerical simulation model of the joint is constructed using the Hashin damage failure initiation criterion based on stress analysis and the equivalent variable stiffness degradation theory based on fracture energy. The effectiveness of the model is verified by tensile tests (peak load error <10%, failure displacement error <10%, and hole diameter deformation error <10%). The influence of layup method, hole size, and laminate thickness on joint performance is explored using the controlled variable method. The results show that the layup method significantly modulates the failure behavior. The 0/90_5s layer, with the highest 0° layup ratio (67%), achieves the maximum peak load (5.77 kN); the 45/−45_5s layer achieves the maximum failure displacement (4.55 mm). The hole size determines the contact damage mechanism. The 4.14 mm hole size, due to the lack of initial clearance, causes stress concentration on the hole wall, leading to large-area fiber/matrix breakage. The 4.24 mm hole size, with a moderate clearance, allows for uniform filling of the pores during rivet formation, optimizing stress distribution and increasing the load-bearing capacity by 16.7% compared to 4.14 mm. The thickness of the CFRP laminate affects the load transfer efficiency. The 3.00 mm thickness achieves optimal stiffness and strength with full hole wall bearing capacity. The 3.60 mm thickness, due to the reduced locking ability of the rivet bulge, causes premature rivet tilting, resulting in a 13.5% decrease in load-bearing capacity compared to 3.00 mm. This study provides some guidance and simulation basis for the parameter optimization design of CFRP/Al countersunk hole blind riveting structures in aerospace composite materials.