The pressure of tritium gas vs. time curves of LaNi3.70Al0.75Mn0.55 alloy absorbing tritium in the constant volume condition at room temperature were studied. Mechanism of tritium absorption and their effect on the tritium absorption rate were analyzed. The results indicated that the pressure decreases rapidly in the initial stage of absorbing tritium, and the T/M can reach to 1.5 std.cc/g in 15 s, the highest tritium absorption rate reaches to 0.28 std.cc·g-1·s-1 maxed out at the α+β phase. The dissociation and chemical adsorption of tritium on the surface of the alloy are the speed control steps at the initial stage of tritium absorption. When the T/M > 15 std.cc/g, the reaction results in phase transition to form tritium compounds, the control steps of tritium absorption is nuclear growth.