Three types of FTIR spectrometers were employed to measure the mid-infrared (4000—400 cm-1) absorption spectra of geometrically frustrated hydroxyl cobalt chloride Co2(OH)3Cl, and the intrinsic absorption peaks in the functional group region and fingerprint region were selected and assigned to corresponding vibrational modes according to its known crystal structure. In the assigning process, great emphasis was laid on analyzing the exact experimental data of hydroxyl stretching vibration mode vOH, that is, estimating the free vOH of the Co3—OH group without any hydrogen bond (H-bond), to obtain the red-shift which reflects the formation of an H-band in Co2(OH)3Cl. A 156 cm-1 red-shift is obtained theoretically which demonstrates the presence of non-negligible weak H-bonds, and eventually result in the discovery of the rarely reported trimeric H-bond in the field of crystalline materials, which consists of three independent hydroxyl donors and only one Cl- acceptor. We explained the relative weakness of this kind of hydrogen bond which may have a critical effect on the lattice symmetry and magnetic structure.