The work aims to explore protective mechanism of grapheme-based protective coating/carbon steel system in sedimentary water of crude oil storage tank. With the sedimentary water of actual oil storage as corrosive medium, and self-made graphene primer and graphene finish paint as protective coating system, electrochemical impedance spectroscopy, potentiodynamic polarization curves in combination with salt spray test were used to investigate corrosion behavior and failure derivation mechanism of graphene coating system in sedimentary water. The graphene primer had certain protective effect on carbon steel in initial period of soaking. Water molecules penetrated into the coating gradually and the coating failed gradually as the soaking duration prolonged. The protective effect of the coating on the carbon steel in the sedimentary water could be significantly improved by the combination of the graphene primer and graphene finish paint. After 46 days of soaking, the coating resistance of graphene composite coating was still 162 MΩ?cm. The graphene primer + graphene finish paint coating system has excellent protective effect on oil tank baseplate in the sedimentary water. The study results have theoretical guiding significance on coating material selection of oil tank baseplate.