Water films are always formed on the outer or inner surfaces of materials in industrial processes and some key parts of instruments because of complex working environments (moisture, hidden water). This situation may lead to inaccurate measurement results or even unsafety process for instrument operation. Therefore, it is important to reliably detect and quantify the formed water films. A coplanar capacitive sensor and an AD7745-based measurement system were designed for the first stage of this study. A glass tank and a specific power ultrasonic humidifier were then added to construct the experimental platform. To verify the feasibility of the designed coplanar capacitive sensor, the thickness of the water films was set to change from 0 mm to 1.5 mm with a 0.1 mm interval. Experimental results based on the self-developed system were compared to the results simulated by COMSOL, and the results derived from Agilent 4294A (an impedance analyzer). The results indicated that the coplanar capacitive sensor could be used to detect and evaluate the thickness of the water film. The resolution of AD7745-based measurement system for water film thickness detection is less than 0.1mm.