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The SiC/Al–Mg–Si composite was obtained by in-situ synthesis, and the effects of Mg addition were investigated in this process. Mechanism of in situ SiC growth in the Resin–Al–Mg–Si system was investigated by fabricating and characterizing a SiC/Al–Mg–Si composite under moderate conditions. X-ray Diffraction (XRD), Scanning Electron Microscopy (SEM) and Transmission Electron Microscopy (TEM) were used to analyze the phase and its microstructure, with the aim of investigating the evolvement of grains in the alloy matrix. Mg2Si formed under higher Mg added conditions, which decreased the molten liquid and Si solute concentrations. Tiny amounts of MgO and MgAl2O4 proved that the pyrolysis gas of resin reacts slightly with the matrix. The key conditions of in-situ synthesis SiC were found that the Mg addition and the Si concentration in the molten alloy were 3%–5% and over 17. 27 % respectively. Ostwald ripening was used to explain the decrease in tiny SiC grains at elevated temperatures. In situ SiC growth is controlled by the interfacial reaction-diffusion mixed model through two different routes in the molten alloy. Two modes of in-situ synthesis were suggested. The intermediate product and template for in situ SiC growth are Al4C3, and Si was shown to diffuse into its particles gradually. Flowing Si in molten alloy reacted with pyrolysis C directly. Specimens validated the peak bending strength with a rise in temperature. The maximal bending strength was 251.89 MPa.