MgH2-LiBH4 system is one of the promising hydrogen storage materials. In the system, it was found that there was a mutual interaction between the two hydrides, but its mechanism has not been clarified yet. In this work, we found an “H-D” exchange between MgD2 and LiBH4 during heating. IR absorption spectroscopy revealed that the peak of “B-D” vibration appeared at 275 °C below the melting and hydrogen desorption of the system, indicating that this exchange proceeded even in solid phases. The hydrogen desorption properties of the composite of catalyst-doped MgH2 and LiBH4 under inert gas were investigated by mass spectroscopy. The results showed that the hydrogen desorption temperature of the first step was over 300 °C, in spite of catalyst-doped MgH2 can release hydrogen at 200 °C. The above results might suggest the hydrogen desorption from catalyst-doped MgH2 is somehow suppressed by the exchange effect between MgH2 and LiBH4.