Two coaxial couplers have been successfully designed and fabricated by Peking University for the BISOL (Beijing Isotope-Separation-On-Line neutron-rich beam facility). These devices use double-cylinder ceramic windows for the lower-frequency couplers. To reduce expenses, they were designed to transmit RF power for the HWR cavities at a level of 20 kW at 162.5 MHz and for the QWR cavities at between 1–5 kW at 81.25 MHz. RF conditioning and long-time testing are necessary steps to verify the couplers’ performance before installation. This paper describes the process and method of high-power conditioning. We completed the high-power conditioning of the couplers at the design power of 20 kW in the continuous wave mode. The devices were tested at room temperature for about 24 h. The results showed that the double-cylinder-window couplers have excellent performance.