La_0.3Y_0.7Ni_3.4-xMn_xAl_0.1(x=0-0.5) hydrogen storage alloys were prepared by vacuum arc melting followed by homogenized annealing. Effect of Mn element on the microstructure, hydrogen storage behavior and electrochemical properties were systematically investigated via different methods. The results show that the microstructure of the annealed alloys closely relates to the Mn content. Higher Mn content facilitates the formation of Ce₂Ni₇ type phase until single phase structure of Ce₂Ni₇- type forms in the alloys with x≥0.3. With the increment of Mn content, the unit cell parameters (a, c) and unit cell volume (V) of Ce₂Ni₇- type phase increase, resulting in the hydrogen absorption platform pressure of the alloys decreasing from 0.079 MPa to 0.017 MPa and the hydrogen storage capacities reaching 1.268wt%-1.367wt%. The electrochemical properties are significantly improved with the addition of Mn. La_0.3Y_0.7Ni_3.25Mn_0.15Al_0.1 alloy exhibits the highest discharge capacity (390.4 mAh·g ^-1). The capacity retention S₁₀₀ of the alloys with x=0.15 and 0.5 are 86.03% and 88.01%, respectively, presenting good cycle stability. Meanwhile, high rate discharge ability (HRD₉₀₀) of the as-prepared alloys is 71.53%-87.73%. It is shown that electrochemical reaction kinetics of the alloy electrodes is controlled by both the electron transfer at the electrode/ solution interface and the diffusion of hydrogen atoms in the alloy bulk.