Abstract: To address the demands of marine engineering construction in China and implement the national “Dual Carbon” strategy, a novel low-carbon marine aggregate concrete (LMC) was developed in this study by incorporating seawater, sea sand, coral aggregate, and low-carbon supplementary cementitious materials (including slag and fly ash). Uniaxial compression tests were conducted to investigate the effects of key mix proportion parameters, curing conditions, and curing ages on the mechanical performance of LMC. The stress-strain relationship of LMC under uniaxial compression was analyzed, and a constitutive model for LMC was subsequently established. Based on mesoscale numerical simulation, the internal stress distribution, damage evolution, and crack propagation processes were explored. Combined with multi-scale analysis covering macro-, meso-, and micro-scales, the failure rules and damage mechanisms of LMC were revealed. The results indicate that the optimized LMC (M_s = 1.46, N/B = 9%, C_s = 0.76) achieves a 60-day compressive strength of 41 MPa under standard curing. The proposed constitutive model agrees well with the interfacial transition zone, ultimately forming a shear-dominated through-going failure zone. The primary hydration products are C-(A)-S-H gels with secondary N-A-S-H gels, which densify the matrix, strengthen interfacial bonding via micro-aggregate filling, and thereby enhance the mechanical performance of LMC.