In the filter rod capsule and cigarette capsule production process, the capsule defects, which include breakage, loss and position offset, occur easily because of the technology of capsule filling, filter rod cutting, and cigarette winding. Because the capsule is buried in the filter rod or cigarette, it is difficult to visually judge its normality without breakage. However, the dielectric constant of the capsule is different from that of the filter rod or cigarette. The difference of their signal will appear in the area whether there is a capsule or not. According to the principle of dielectric constant detection in microwave resonator, the signal of filter rod in the length direction was collected by a microwave resonator sensor. The leakage, location, and breakage of the capsule in the filter rod were detected according to the difference of the signal intensity. A detector was designed based on the microwave resonance cavity perturbation technique. The detector was mainly composed of a filter rod feeding device, a filter rod conveying device, a microwave detection unit and a sorting device. The designed detector and the established algorithm featured high accuracy, which was 0.1 mm for detecting the position of the capsule, and good recognition effect for defects or absence of the capsule. The detector provides a rapid and accurate quality detection method for capsuled-filter rods.