Effects of silicon powder additions on the phase composition and microstructure of silicon- corundum-silicon nitride composite refractories were investigated guided by the theory of metal transient plastic phase and the Si-O-N phase diagram. On this basis, silicon-corundum-silicon nitride composites were prepared under air atmosphere. The results showed that the addition of silicon powder could reduce the width of the oxidized film and improve the densification; silicon addition could decrease the oxygen partial pressure in the specimens and contribute to the sintering of the materials. Part of silicon could remain in the materials. The industrial experiments suggested that silicon powder mostly filled in the interstices of the matrix of the sintered products with addition of less than 8 wt.% Si powder, and with increasing Si powder additions the densification of the product was improved, cold strength increased, and hot strength also increased due to the in-situ formed fibrous SIALON by silicon powder; silicon powder formed continuous phase which led to the decrease in densification and hot strength when more than 8 wt.% silicon powder was added.