The invention provides a mixed particle system for sewage treatment and a method thereof. The mixed particle system for sewage treatment comprises a contactor, wherein the interior of the contactor is provided with a gas-liquid-solid three-phase zone which is composed of a gas phase, a liquid phase and a solid phase; the liquid phase is a continuous phase; and the solid phase is mixed particles composed of light particles and heavy particles. According to the invention, the light and heavy mixed particles are added into a gas-liquid-solid fluidization system; the light particles carry out downward fluidization while the heavy particles carry out upward fluidization under the action of gas and liquid, so the solid particles are uniformly distributed along a vertical direction; and thus, microbes adhered onto the outer surfaces of the light and heavy mixed particles are allowed to fully contact with sewage so as to promote biochemical reactions and save energy.