The FRP-reinforced seawater sea sand concrete structure prepared by fiber-reinforced composite (FRP) bars to replace the traditional steel bars can effectively solve the problems of the lack of river sand resources and the corrosion of steel bars in marine concrete structures. However, fiber-resin bond degradation and resin matrix hydrolysis may still occur in FRP in a highly alkaline environment, which may lead to the degradation of the mechanical properties of FRP bars. In this paper, a PZT-BFRP degradation monitoring sensor is developed based on electromechanical impedance (EMI) technology. The degradation process of the interlayer shear strength of BFRP bars and the change of the monitoring signal of the sensor are analyzed through the experiment. The results show that under a high temperature and alkaline environment, with the increase of corrosion age, the interlaminar shear strength of BFRP first increased slightly and then continued to decrease. The characteristic peak frequency and some statistical damage index defined based on the conductance show a monotonically increasing or decreasing change. By establishing the regression equation between the signal of the sensor and the mechanical properties of BFRP, the quantitative prediction of the deterioration degree of BFRP bars can be realized.