We perform theoretical investigation on mechanisms of LiD/LiH/LiT+H2O reactions with second order perturbation theory (MP2). Calculations of enthalpy change and reaction rate constant are in agreement with experimental data and other theoretical values. Results show that two reaction channels exist for LiD+H2O reaction, channel 1 is the main reaction channel. Kinetic Isotope Effect (KIE) is not so large however fracture of isotope bond takes place in the reaction, which is noticeably different from theoretical maximums of primary isotope effect. Individual contributions of KIE effect are listed as follows ηvib>ηrot>ηtun>ηpot>ηtrans, indicating that main contribution of isotope effect results from vibration factor.