To realize precise and non-destructive determination on moisture content of plant leaves, a moisture detector based on leaf capacitance was designed. Circular parallel plate capacitor was used to detect capacitance values of plant leaves, FSR402 pressure sensor was used to detect pressure on plant leaves given by circular parallel plates, and MSP430 single-chip microcomputer was used as micro-controller. Maize leaves were used as samples to study the influence of moisture content and pressure on obtained capacitance values, and the best pressure for maize leaves’ capacitance determination was obtained. The model describing the relationship between capacitance and moisture content of maize leaves at the best pressure was established and verified. The results showed that the capacitance values of maize leaves increased with moisture content and pressure. The best pressure for obtaining capacitance values of maize leaves was 4 N. The running test indicated that the absolute measurement error of the designed detector for moisture content of maize leaves was -1.2%~1.7% in wet basis when the moisture content was between 55% and 80%, and the response time was within 3 s at oscillation frequency of 8 MHz.