This work successfully developed a novel high-performance detector based on rare-earth-doped cesium lead chloride (CsPbCl₃) inorganic scintillation crystals, targeting the critical demand for GHz-rate capabilities in ultrafast radiation detection at advanced light sources. The Ba²⁺-doped CsPbCl₃ crystals, grown via the vertical Bridgman method, exhibit sub-nanosecond fluorescence rise times, with the pure crystal measuring ~209.6 ps and optimized doped crystals achieving ~50-75 ps. The crystals also feature nanosecond-scale decay times and enhanced light yield through defect engineering. By integrating this core scintillator with a microchannel plate photomultiplier tube (MCP-PMT) featuring sub-nanosecond transit time and a self-developed 2.5 GHz high-speed acquisition system, a complete ultrafast detection system was constructed. Rigorous testing using an optically generated equivalent GHz pulse train demonstrated that the system can clearly resolve consecutive fluorescence pulses with an average peak interval of only 0.79 ns, successfully achieving a high-repetition-rate detection capability of 1.26 GHz. Field application at the Shanghai Synchrotron Radiation Facility's soft X-ray free-electron laser (SXFEL) showed that its X-ray pulse response width is narrower than 4 ns, far superior to the >24 ns response of a reference LYSO:Ce crystal. These results validate the detector's exceptional sub-nanosecond time resolution and GHz-rate pulse discrimination, providing a reliable technical solution for ultrafast time-resolved diagnostics and photon beam loss monitoring in next-generation scientific facilities.