Glass fiber reinforced polymer bars have potential application in the seawater sea sand concrete. In this study, an analytical method was proposed to examine the internal crack development in glass fiber reinforced polymer–sea sand concrete composites. The method was derived by analyzing the strain dissipation and shear-lag behavior. Thus, the internal cracks can be quantitatively identified by the proposed model with Fourier transform analysis. The proposed approach was experimentally verified through pullout tests for the specimens with artificial cracks. The results indicated that the prediction errors were less than 10%. In addition, the piezoelectric sensors were adopted to verify the proposed model. A damage index (DI) defined by the energy loss of received signals was used to characterize cracks inside the composite. Loading test results indicated that the DI-calculated cracks were consistent with the proposed analytical model. Finally, the cyclic loading tests of the glass fiber reinforced polymer composite were conducted using both strain gauges and PZT sensors. The results have shown that the differences between the proposed approach and PZT sensors were less than 15%. Thus, the proposed approach in this study could provide an alternative approach for quantitative evaluation of internal cracks in the glass fiber reinforced polymer–coral sand–cement composites.