The 100-m X-ray Test Facility (100XF) was developed according to the calibration requirement of the first X-ray astronomical satellite Insight-HXMT in china. After that, the 100XF has provided calibration services for other X-ray astronomical satellites. In order to test the energy response matrices of space-borne detectors and to extend the capability of 100XF, a monochromator based on channel-cut crystals is developed. So far, 3.0-20.7 keV monochromatic X-rays have been realized successfully with Si(111) crystal. The monochromaticity (Δ E FWHM /E) is better than 0.6% @5.9 keV, the flux stability can reach 2.6% in about one hour. The energy stability is particularly good, the variation is 0.05% in about one hour. Further more, the beam spot spreads in the plane orthogonal to the rocking direction after a propagation of 100-meter, which is sufficient and useful for testing detectors with large sensitive area. In this paper, we present the research and establishment of the X-ray monochromator at the 100XF. [ABSTRACT FROM AUTHOR]