Abstract: This study investigates the deformation and fracture mechanisms of TC4 titanium alloy for deep-sea submersible structures with bimodal and lamellar microstructures under different stress states. Compression, pure shear, combined tension-shear, and uniaxial tensile specimens were designed to establish representative loading conditions. Load-displacement testing, fracture surface analysis, grain orientation and geometrically necessary dislocation (GND) density characterization, and transmission electron microscopy (TEM) observation of dislocation configurations were conducted. In addition, a pressure- and Lode angle-dependent elastoplastic damage model, implemented through an Abaqus-VUMAT subroutine, was employed to analyze the differences between the two microstructures in terms of macroscopic load-bearing response, fracture morphology, strain localization, and dislocation evolution. Numerical validation was further performed for damage initiation and fracture propagation in bimodal TC4 titanium alloy.The results show that the two microstructures exhibit similar responses in the elastic stage, whereas their plastic load-bearing capacities vary with the stress state. Under normal-stress-dominated conditions, such as uniaxial tension and combined tension-shear loading, the bimodal microstructure exhibits a higher fracture load. In contrast, under shear-stress-dominated conditions, such as compression and pure shear loading, the lamellar microstructure shows a higher ultimate load and stronger sustained load-bearing capacity. Microstructural characterization reveals that the bimodal microstructure forms relatively uniform dimple morphology and GND distribution during tension-dominated deformation, which promotes plastic compatibility and delays localized damage. The lamellar microstructure, under shear-dominated deformation, develops directional shear dimples, continuous high-density dislocation bands, and dislocation pile-ups near the α/β lamellar interfaces, thereby enhancing work hardening and shear load-bearing capacity.The finite element simulations reasonably reproduce the load-displacement responses, damage localization, and fracture paths of bimodal TC4 titanium alloy under different stress states. The deformation and fracture behaviors of TC4 titanium alloy with bimodal and lamellar microstructures are jointly governed by stress state, interfacial constraint, dislocation evolution, and strain partitioning. These findings provide a basis for microstructural selection and failure prediction of TC4 titanium alloy under complex loading conditions.