A technique for material density microware detecting is introduced in this paper. Using a microwave resonant technique, by detecting the change of resonant frequency and the attenuation of microwave magnitude, the density of the material in the microware cavity resonator can be measured. Measurement principle, calibration technique and measurement process are mainly discussed. The calibration process is emphatically illustrated, that is, the resonant frequency of the cavity resonator is approached by using frequency sweep method, and some formulae are deduced. After data analysis and process of the experiment, the formula for the density of the material in the microware cavity resonator is given. The error of this technique for microware density detecting can satisfy the design requirement.