The principle of the moisture measuring with microwave drying was analyzed theoretically, and the microwave heater, the power supply system, the high precision weight sensor and the single chip microcomputer (SCM) system were all designed. The software and hardware of the moisture measuring instrument were developed in the lab. Through the experimental data, it takes only about 4 minutes to measure the moisture ratio with the microwave moisture measuring instrument, and the measurement errors are less than 0.05%. Compared with the traditional moisture measuring instrument using microwave attenuation method, the new instrument with microwave drying can detect the moisture of solid material rapidly with high precision.