Abstract: Glass-fibre reinforced polypropylene (GFRPP) composite rod integrate the advantages of thermoplastic resin multi-molding, environmental friendliness, recyclability, and high strain and low cost of glass fiber. In the field of concrete structures, GFRPP rods are expected to replace steel bars and thermoset fibre reinforced polyer (FRP) bars as a new composite material. In this paper, accelerated experiments were used to study the long-term evolution of water absorption, interlaminar shear properties and degenerative mechanism of GFRPP rod under simulated seawater and sea sand concrete environment. The results show that the water absorption behavior of GFRPP rod conforms to Fick's law, and the saturation water absorption rates of GFRPP rods at immersion temperatures of 21°C, 40°C and 60°C are 0.63%, 0.78% and 0.81%, respectively. After 120 days of immersion in 21°C, 40°C and 60°C simulated seawater sea sand concrete pore solutions, the shear strength retention rates between GFRPP rods are 80.5%, 72.8% and 66.5%. Finally, the performance degradation mechanism of GFRPP rods under simulated seawater-sea sand concrete pore solution immersion was revealed by combining SEM and FTIR characterization techniques.