A concrete cold joint is a weak interface within structures, and is prone to shear failure. Additionally, as potential pathways for external corrosive ions, cold joints experience interface performance degradation in adverse environments, thereby compromising the durability of the structure. This study utilized molecular dynamics simulations to analyze the shear behavior of two models of concrete joints: the CSH-a-to-CSH-b (CC) interface and the CSH-a-to-SiO2 (CS) interface, based on calcium silicate hydrate (CSH) and silicon dioxide (SiO2) as substrates. The study found that the penetration ranges of water into the CC and CS interfaces were 17Å and 10.5Å, respectively. When exposed to a saline-alkaline environment, the penetration range of Na2SO4 solution increased by 26.5 % and 185.7 % compared to the humid environment for the CC and CS interfaces, respectively. Furthermore, corrosive environments influence ion interactions at the interfaces, leading substrate ions to react preferentially with ions in the corrosive solution, thereby weakening the bonding performance of the interface. The effect of environmental conditions on the shear performance of joints is ranked as follows: saline-alkaline > humid > dry. Under dry conditions, the maximum shear stress of the CC and CS interfaces reached 0.93 GPa and 0.88 GPa, respectively. In humid and saline-alkaline environments, the maximum shear stress of the CC interface decreased by 38.7 % and 68.8 %, while that of the CS interface decreased by 31.8 % and 62.5 %. Additionally, shear failure at the interfaces consistently occurred in regions where bond energy was unstable. This study provides atomic-scale insights into the degradation of concrete cold joints, guiding material design and interface optimization in engineering practice.