•Building a hybrid-polarization GPR system for rebar corrosion detection.•Perpendicular polarization against rebar orientation is more sensitive to rebar corrosion.•Classifications by H-Alpha polarization decomposition shift in corrosion process.•Polarimetric GPR shows potential application in evaluation of early-stage corrosion. As a non-destructive testing method, ground penetrating radar (GPR) has been applied to monitor the corrosion of reinforced bar (rebar) in concrete. However, a traditional GPR system employs a pair of antennas, and can only measure single-polarization radar signals. In this paper, a hybrid-polarization GPR system is proposed for detection and evaluation of rebar corrosion. It can record rebar reflections in two orthogonal polarimetric channels, from which a full-polarimetric scattering matrix can be estimated through polarimetric decomposition and calibration. An accelerated corrosion experiment was conducted and the results prove that radar signal in the polarization channel perpendicular to the rebar orientation is more sensitive to the corrosion, compared with the commonly-used parallel polarization channel. Analysis of the scattering mechanism through H-Alpha decomposition shows that the corroded rebar shifts from the low-entropy dipole scattering to the low-entropy surface scattering. By filling the migrated GPR image with the color of the H-Alpha scattering classification, it is able to visually evaluate the early-stage corrosion of rebar in concrete.