The time-varying properties and improvement mechanism of hybrid FRP (HFRP) bars in simulated seawater sea sand concrete environment through tensile testing, water absorption analysis, digital microscopy and X-ray CT. The results demonstrate that hybrid carbon fibers significantly reduces moisture absorption in HFRP bars compared to basalt FRP (BFRP) bars, attributable to the superior mechanical properties and chemical stability of carbon fibers. Temperature exhibited a pronounced influence on the tensile properties of HFRP bars, and the increase in hybrid carbon fiber volume fraction (VCF) alleviates the degradation induced by high temperature and alkaline pore solution environment. When the VCF is 25%, the tensile strength retention exceeded 43% after 90 days of corrosion in a 55 °C pore solution. The failure mode of HFRP bars is affected by the VCF value, and the BFRP/CFRP interface is also a weak link in bars.