The crosstalk of scintillation screen signals is the main factor that affects the spatial resolution of X-ray detectors. The spatial resolution of fiber-coupled GAGG_Ce single crystal scintillation screen CCD/CMOS detector is studied based on the point spread function theory. The ray crosstalk of GAGG_Ce single crystal scintillation screen and fluorescence crosstalk are simulated by Monte Carlo program EGSnrc and optical simulation software Zemax respectively. The simulation results show that fluorescence crosstalk is the most important factor affecting the spatial resolution of the detector for low-energy X-ray radiation imaging. In addition, the method of suppressing fluorescence crosstalk by reducing the numerical aperture of optical fiber panel is studied, and the relationship among numerical aperture of optical fiber panel, detector spatial resolution and X-ray conversion factor is obtained. The correctness of the simulation results is verified by the self-made CCD detector test.