Infrared transmission measurements were made to investigate the influence of hydrogen passivation on Hg_(1-x)Cd_xTe wafers. It was observed that, after boiling the samples in water, the infrared transmittance increased, the absorption edge was moved to the short wave direction and the absorption below the energy gap was reduced, and that the range of infrared transmission was narrowed. Through calculating the trans-mittance of Hg_(1-x)Cd_xTe with the multiple layer model, it was found that hydrogen passivation influences not only the surface but the whole bulk of Hg_(1-x)Cd_xTe wafer. Careful analyses showed that, the changes of Hg_(1-x)Cd_xTe infrared transmission spectrum after hydrogen passivation are the results of the effective passivation of residual impurities or defects, the reduction of carrier concentration, the increase of composition x, the effect of local internal electric field of charged impurities or defects, and the enhancement of scattering of charged impurities or defects.