The wall-thinning defect is one of the critical flaws that have been posing a severe threat to the structural integrity of Glass Fibre Reinforced Polymer (GFRP) employed in the hostile environment. In this paper, a sweep-frequency microwave testing method based on Cross-polarization Microwave Reflectometry (CMR) is proposed for detection and imaging of subsurface wall-thinning defects in the unidirectional GFRP. The simulation model of CMR is established along with the scrutinization of the characteristics regarding the testing signal in Ka band (26.5 GHz∼40 GHz). In experiments, a microwave testing system is built up for inspection of GFRPs together with the CMR probe designed using a pair of Ka-band rectangular waveguides based on the simulation model. Imitative subsurface wall-thinning defects and an actual impact defect in GFRP samples are inspected by using the system, and further imaged by using the time-domain signal and Range Migration Algorithm (RMA). The superiority of the proposed method is quantitatively identified through comparison of the signal-to-noise ratio of the imaging result between the CMR and the single-polarization microwave reflectometry.