In this paper, Si3N4 composite ceramic were synthesized with kerf loss silicon waste powder as the raw material by molten salt assisted nitridation method. Na3AlF6 molten salt was introduced into the system and the effects of Si-salt ratio, nitridation temperature on the nitriding product were investigated. These results indicated that the addition of Na3AlF6 promotes the conversion of waste silicon to Si2N2O, and 90.7% of Si2N2O phase was obtained with the Si-salt ratio of 1:1.5 at 1350℃. The increase in temperature causes the main phase of the product changes from Si2N2O to β-Si3N4, which is due to the decomposition of Si2N2O and the α→β phase transition of Si3N4. And the content of β-Si3N4 phase reached 72.9% with the Si-salt ratio of 1:0.75 at 1450℃. The formation mechanism was analyzed combined with the in-situ nitriding process.