Concrete structures in marine environments will be attacked by corrosive ions in seawater, including chloride ions, sulfate ions, etc. Those ions can significantly cause the degradation of the mechanical properties and durability of concrete. They can also cause problems like reinforcement corrosion, cracking, and spalling, and lead to the decrease of the service life of concrete structures and pose serious threats to structural safety. Investigating the evolution of mechanical properties of concrete under the combined action of sulfate and chloride ions is crucial for designing more durable concrete structures in marine environment. However, there is currently no consensus on the evolution of concrete modulus under such combined corrosion conditions. To address this gap, this paper presents an experimental study on the modulus evolution of concrete under the combined corrosion of sulfate and chloride ions. Concrete samples were prepared and subjected to accelerated corrosion experiments in artificial seawater. Ultrasonic non-destructive testing (NDT) was used to measure the changes in ultrasonic wave velocity in concrete. This allowed us to track the evolution of concrete modulus under corrosion conditions. Based on the experimental results, a mechanical-chemical model was developed. The model integrates the continuous hydration of concrete, the chemical reactions of sulfate ions with concrete, the complexation reactions of chloride ions, and other chemical processes. The model helps explain the competitive mechanism between sulfate and chloride ions during the corrosion process. The results show that the elastic modulus of concrete initially increases due to the filling effect of hydration products and the formation of ettringite and Friedel's salt. However, as the corrosion continues, the excessive filling of pores by expansive products caused by sulfate ions leads to a gradual decrease in modulus. The model successfully captures these changes and fits well with the experimental data. Additionally, it was found that chloride ions react with tricalcium aluminate to form Friedel's salt. This reduces the amount of ettringite formed by sulfate ions, thereby reducing the expansion force and delaying the decline in dynamic elastic modulus. This also reduces the damage caused by the expansion force. The findings provide a theoretical basis for designing concrete structures in marine environments with stronger resistance to sulfate and chloride corrosion. This can help engineers develop more effective strategies to enhance the durability of concrete in marine environments. This, in turn, can extend the service life of marine infrastructure and reduce maintenance costs.