The present invention discloses a method of using a waste silicon slurry. The method includes the steps of: (A) obtaining a waste silicon slurry containing a cutting oil and a metal; (B) treating the waste silicon slurry with a first reagent for reacting with the cutting oil; (C) treating the waste silicon slurry with a second reagent for reacting with the metal; (D) separating products resulting from step (B) and step (C) to obtain a solid portion; and (E) treating the solid portion with a third reagent to obtain products, including silicates and hydrogen gas.