A slightly heavy particle system for wastewater treatment includes a contactor. The contactor includes a gas-liquid-solid three-phase region, and the gas-liquid-solid three-phase region includes gas phase, liquid phase, and solid phase. The liquid phase is the continuous phase and the solid phase is slightly heavy particles. The slightly heavy particles are able to carry some microorganisms on their surfaces at least, and the density of the slightly heavy particles is higher than the density of the liquid. The slightly heavy particle system applied in wastewater treatment can increase the concentration of microorganisms significantly, improve the ability to bear impact load, produce less sludge, and without sludge expansion. Meanwhile, suspending slightly heavy particles with the combined effect of gas and liquid is able to reduce energy consumption to a larger extent. Therefore, this system features with high efficiency and low energy consumption.