This paper presents the optimal design of a small-sized sensor, high sensitivity, and low cost to apply in moisture measurement of thin paper film. The capacitive sensor is designed based on a printed circuit board (PCB) with a charge amplifier circuit to reduce noise. The thin paper film is placed onto the sensor surface. When the moisture of the thin paper film changes, the dielectric will change, leading to a corresponding change in the sensor's capacitance. From this change in capacitance can be estimated the change in paper moisture. In this study, the sensor structure and electrode size were calculated and simulated to optimize the signal intensity and penetration depth of the sensor electric field into the moisture measurement paper layer. The proposed sensor can measure the moisture content of thin paper films with a thickness of 0.02 to 0.22 mm with an output capacitance varying from 0.88 to 6.848 pF, corresponding to a thin paper film moisture content of 20 to 50%. Humidity sensors are used in instrumentation, automation systems, agriculture, climatology, and medicine.