To investigate the multiphase coupled flow characteristics and apparent wall slip phenomenon in horizontal pipes, air-hydraulic oil-solid mixtures flowing in 50 mm-diameter and 32 mm-diameter pipes are tested. The results show that the critical liquid velocity corresponding to the transformation of the flow structure is between 0.75 and 1 m/s. Compared to the liquid-solid flow, the injection of gas has a drag reduction effect, to a certain extent. However, with an increase in the superficial gas velocity, the relative slip contribution first increased and then decreased. Moreover, the pressure gradient and wall slip velocity of the gas-liquid-solid coupling flow were sensitive to the superficial velocity of the gas and liquid phase and the phase volume fraction, as well as pipe conditions, including the diameter and roughness. Finally, on the basis of theoretical and experimental data, a wall slip model is proposed to predict the apparent wall slip velocity and pressure gradient in gas-liquid-solid coupled flow. The model shows that the apparent wall slip effect is promoted under the condition of a low volume fraction of the dispersed phase. Compared with the experimental data, the prediction results of the model are acceptable.