The 15SiC_P/2009 aluminum matrix composite is treated by microarc oxidation in silicate electrolyte. Optical emission spectra (OES) in microarc discharge process at different positive voltages are collected by optical fiber spectrometer, and their plasma parameter characteristics at different oxidation time are evaluated. The morphology, composition and phase constituent of oxide films are analyzed by scanning electron microscopy (SEM) with energy dispersive spectroscopy (EDS), glow discharge optical emission spectroscopy (GDOES) and X-ray diffraction (XRD), and the growth model of oxide film is proposed. Al, Cu, Si, C elements from the composite matrix and H, O, Na, Si elements from the electrolyte participate in the microarc discharge process. The plasma temperature in the microarc discharge channels reaches 5000-10000 K, and the electron density is in range of 4.0×10²¹-1.0×10²² m⁻³, which is a localized thermal equilibrium state. As the applied voltage increases, the discharge sparks are enhanced, meanwhile the electron temperature and density increase. The SiC reinforcement particles hinder the growth of oxide film, but the simultaneous high temperature in the discharge channels results in the oxidation of reinforcement particles and the size of these particles gradually decreases. And the high temperature in the discharge channels also enhances the formation of α-Al₂O₃, γ-Al₂O₃ and mullite phases in oxide films.