Abstract: When the oil in crude oil storage tanks is contaminated by water, it can severely compromise the safety performance of oil-dependent equipment. This paper investigates a polymer thin-film capacitive sensor, the working principle of which is based on the change in dielectric constant after water molecules permeate the polymer film and reach equilibrium. Different dielectric constant values correspond to different mass fractions of water in the oil. The study details the design of the sensor′s circuit architecture, signal transmission circuit, and communication protocol. A prototype was developed and subjected to comparative testing. Experimental results demonstrate that the sensor exhibits high measurement accuracy, with minimal deviation from standard testing instruments.