The spectral resolution, spatial resolution of monochromatic X-ray images of a spectrograph based on convex crystal were discussed by ray tracing calculations taking into consideration the effect of rocking curve of the crystal. A mica convex crystal spectrograph was designed and then tested successfully on “QiangGuang-1" pulsed power facility. Time integrated monochromatic X-ray images of Al Z-pinch plasmas showed the X-ray line emissions of K-shell came from the core region of a diameter of about 2.3 mm.