Abstract: Aimed to the problems of difficult installation and low measurement accuracy in high moisture, the grain moisture sensor has improved from the traditional parallel polar plate probe to plane polar plate probe. The plane polar probe with protect pole was used to decrease the affect of the stray capacitance and interference, and some critical parameters of the plane polar probe that influenced sensitivity and accuracy of the sensor were proposed. The finite element method was used to analyze the electric field and capacitance of the plane polar probe. The results showed that the relations between measurement depth of grain, capacitance of plane polar probe and the ratio of the width of probe’s drive-plate to the clearance between two induction-plates, and the optimized size of the probe was obtained when the ratio was 0.9. Experimental result proved that maximum error for measuring moisture of the sensor was ±1.5%, measuring moisture ranged from 6% to 36%, and measuring temperature ranged from -10 to 80℃. This study contributes in optimizing the measurement accuracy and lowering the difficulty of installation of the plane polar probe.