The civil aircraft wastewater system generates negative pressure through the pressure difference between the cabin and the external environment to transport the flushing wastewater from the toilet, lavatory gray water, and galley gray water to the waste tank. The wastewater in the wastewater system pipeline presents a three-phase mixed flow of gas, liquid, and solid. The multiphase flow performance of wastewater systems has a significant impact on passenger comfort during flight. In order to explore the influencing factors of multiphase flow performance in wastewater systems, this paper conducts simulation calculations on the gas-liquid-solid three-phase flow characteristics under different liquid-solid composition ratios and different wastewater pipeline diameters. The results show that when the liquid-solid ratio in the wastewater system branch is greater than 2:1, the larger the liquid-solid ratio, the smaller the average cross-sectional velocity of the gas, liquid, and solid three-phase in the branch, the greater the total pressure loss in the pipeline, and the longer the liquid-solid two-phase flow time. When the liquid-solid ratio is less than 2:1, the larger the liquid-solid ratio, the shorter the solid-phase flow time. The average cross-sectional velocity of the gas, liquid, and solid phases in the pipeline increases with the increase of the pipe diameter, and the flow time of the liquid and solid phases decreases with the increase of the pipe diameter. The branch of the wastewater system with a large pipe diameter can effectively improve the ability of the gas phase to transport liquid and solid phases in the pipeline, reduce the total pressure loss of the system, and reduce the collision between the solid phase and the pipe wall.