The invention discloses a method for preparing an in-situ Al matrix composite. The method comprises the following steps: adding a high-temperature covering agent into pure Al and carrying out smelting, so as to obtain a melt; adding KBF4 and KTiF6 into the melt, and carrying out uniform mixing for a reaction; and after a by-product is removed, sequentially adding an Al-Cu intermediate alloy, an Al-Zr intermediate alloy, an Al-Sc intermediate alloy, an Al-Mn intermediate alloy, an Al-Ti intermediate alloy and pure Mg, adding a harmless Al alloy refining agent, and sequentially carrying out degassing refining and gas atomization at 700-850 DEG C, so as to obtain in-situ synthesized TiB2 particle reinforced Al-Cu-Mg composite powder. Through the method disclosed by the invention, the Al matrix composite powder which can achieve high laser absorptivity and high particle sphericity is prepared; furthermore, the technological operation is simple and easy, the cost is low, and the efficiencyis high; and the method is suitable for mass production.