X-ray imaging plates are one of the most important X-ray imaging detectors and are widely used in inertial-confinement fusion experiments. However, their linear response range, which is the foundation of their quantitative data analysis, has not been sufficiently deeply investigated. In this work, we develop an X-ray fluorescer calibration system and carefully explore the linear response range of X-ray imaging plates. For the first time, nearly the entire grayscale range of the X-ray imaging plate linear response-7819-64 879 in the range of 0-65 535-has been observed. Further, we discuss the uncertainties involved in the calibration process. This work demonstrates the excellent linear response qualities of X-ray imaging plates and provides a significant foundation for expanding their quantitative applied range.