The invention discloses a composite carbonaceous reducing agent for smelting metal silicon and a preparation method thereof. The composite carbonaceous reducing agent comprises purified carbon powder and charcoal powder and/or petroleum coke powder, wherein the weight percentage ratio of the purified carbon powder to the charcoal powder to the petroleum coke powder is 100:(0-40):(40-0); and the purified carbon powder is obtained by separating and purifying the waste cathode after aluminum electrolysis by a flotation method, and contains 80-91% of fixed carbon, 1.5-3% of ash content and 5-7% of volatile content by weight. The preparation method comprises the following steps of: breaking the waste cathode after aluminum electrolysis, and grinding until the particles with particle sizes of -74 microns account for over 90%; adjusting the liquid-solid weight ratio of the slurry to 1:(2-4); adding a collecting agent, an inhibitor and a foaming agent, and separating the purified carbon powder from electrolyte by closed flotation; and uniformly mixing the purified carbon powder, the charcoal powder and the petroleum coke powder according to a proportion so as to obtain the composite carbonaceous reducing agent. Through the invention, the carbon resources can be sufficiently utilized and recycled, 10-30% of charcoal reducing agent can be replaced, the utilization rate of carbon is improved by 10%, and the output and quality of the metal silicon are improved.