The invention discloses an Ag-Mg-Al alloy secondary emission cathode material and a preparation method therefor, and belongs to the technical field of electronic emission cathodes. The method comprises the steps: taking metal silver as a base body, taking the metal elements: Mg and Al as active materials, and preparing an Ag-Mg-Al alloy thin piece through the technologies of induction melting, cold rolling, annealing and polishing; carrying out the thermal activation of the Ag-Mg-Al alloy thin piece in a low-pressure oxygen atmosphere (0.5-50.0 Pa), and obtaining a MgO-Al2O3 composite thin film layer with the thickness from 80-120nm. The novel Ag-Mg-Al alloy secondary emission cathode material is high in secondary emission coefficient, is resistant to electron bombardment, is stable in performance, is simple in preparation process, and is hopefully applied in the fields of photomultipliers and caesium atomic clocks.