Abstract: To address the growing shortage of river sand and freshwater resources in China's coastal regions and the increasing demand for marine infrastructure, it is imperative to clarify the strength degradation mechanism of seawater-sea sand concrete (SSSC) under "high temperature, high humidity, high salt" coupled environments and establish a quantifiable residual strength prediction model. This study employs experimental simulation methods, designing and preparing 57 SSSC cube specimens with variable combinations of two temperatures (20°C, 60°C), three relative humidities (80%, 90%, 100%), and three durability periods (2, 4, 6 months) to simulate different marine service environments such as the atmospheric zone, splash zone, and fully immersed zone. Through standard curing, constant constant temperature and humidity aging, and compressive strength testing, the effects of temperature, humidity, and durability period on the mechanical performance of SSSC were systematically analyzed. The findings indicate that SSSC exhibits a slight increase in compressive strength during the initial service period (2 months) due to continued cement hydration and salt filling of micro-pores; however, as durability extends, strength gradually declines, particularly under 60°C and 100% relative humidity conditions, where the compressive strength decreases by 17.43% after 6 months. Finally, based on experimental data, a relative compressive strength prediction model applicable to environments with 80%, 90%, and 100% relative humidity was established, providing a theoretical foundation and data support for the full-life-cycle design and safe application of coastal SSSC structures.