The invention relates to an aluminum matrix composite, in particular to a preparation method and device of an in situ aluminum matrix composite. The preparation method of the in situ aluminum matrix composite is characterized by comprising the steps that firstly, under the protection of gas, reactants are involved into melt layer by layer for a full contact reaction by means of electromagnetic stirring and interface reverse shear stirring formed by a graphite rotor; then the graphite rotor is placed into the reacting melt for continuous reverse stirring; and finally, casting molding is conducted at a normal casting temperature. The preparation method has the characteristics that composite reinforcement bodies are high in reacting generation efficiency and even in distribution; and the composite is stable in performance; the preparation method is easy to achieve; and industrial application is facilitated.