Calibration is one of the key issues for the Hard X-ray Modulation Telescope (HXMT) on board the first Chinese astronomical satellite. As its core sciences are to observe the Galactic X-ray binaries, transients and the diffuse X-ray emission, the corresponding calibration tasks are mainly about the energy response and the effective area. To this end we are building two facilities specified to the calibration needs of the three scientific payloads of HXMT: the high energy telescope (HE), the medium energy telescope (ME) and the low energy telescope (LE). By adopting a double crystal monochromator, the X-ray beam can be extracted with an intrinsic energy dispersion of well below 1%. The light spot is limited to a size of a few mm under the constraints of the length of the facility which is about 6 m, and of the requirement on the intrinsic energy dispersion which is less than 1%. The HE facility was accomplished in 2013 and the LE/ME facility will be available in the middle of 2014.