In order to solve the problem that continuous online single-capsule testing of capsule-containing rolled products cannot be performed during the rolling process, according to the operating characteristics of the rolling machine, an online capsule detection device for the rolling unit was designed. The device is mainly composed of a capsule detection unit, a display operation unit, a fault diagnosis unit and a data communication unit. The system first generates a terahertz signal source and collects the terahertz signal generated after passing through the capsule in the cigarette filter to be detected, performs real-time calculation through the detection processing unit, and aligns the processed signal with the cigarette through the axis encoder, and finally rejection through the rejection valve. The ZJ17 rolling unit and cigarette products with a diameter of 5.4mm, a capsule diameter of 2.8mm, and a filter length of 30mm were tested. The results showed that the online unit of the rolling unit was used to detect the presence or absence of capsules and damaged capsules. The continuous detection accuracy of the branch is 100%, and the accuracy of the continuous detection of the online single branch for the capsule displacement of ± 4mm or more is 85%. The device realizes the online safety inspection of capsules containing rolled products containing capsules, and opens up new ideas for the quality assurance of capsules containing rolled products.