The invention provides an AlMgSi alloy sheet and a preparation method thereof. The sheet comprises the following components by mass percent: 0.4-0.8% of Mg, 1-1.5% of Si, 0.7-0.8% of Cu, 0.3-0.4% of Mn, 0.4-0.6% of Fe, 0.1-0.2% of Zr and the balance Al. The process for preparing the sheet comprises the following steps: smelting, blank preparing, hot rolling, cold rolling, annealing heat treatment,solid solution heat treatment and aging heat treatment. The invention has the following advantages: the elements for preparing the alloy have few types, most of which are conventional metal elements,therefore, the alloy is low in cost; the alloy smelting process is simple, and the reasonable rolling process and heat treatment process ensure the ultimate tensile strength of the alloy to reach over 390Mpa; cold rolling is adopted in later stage of rolling, thereby dispensing with heating in the hot rolling process and lowering the energy consumption.