The prediction of interface shear stress is one of the great challenges of stratified gas-liquid flow in a horizontal pipe. In this work, a new method is proposed to predict interface shear stress, in which the gas-liquid interface is regarded as a flat specified shear wall (SSW). The gas flow is numerically simulated under the same conditions as those in Strand's experiments, the gas-wall and interface shear stress correlations are then modified, and a new Kowalski-type equation of liquid-wall shear stress is presented. Different models are adopted to obtain the gas flow velocity and gas-wall, interface, and liquid-wall shear stresses, and the results are compared with the experimental data and analyzed. The results show that the gas-wall shear stress is well predicted by the SSW k-omega model, the interface shear stress is well predicted by the SSW k-epsilon model, and the liquid-wall shear stress is well predicted by both the two-fluid model and the Kowalski-type equation. The predictions of liquid holdup and pressure drop of the SSW method improved and agreed better with the experimental data when the gas Reynolds numbers were 9000 <= Re-G <= 50000 and the liquid Reynolds numbers were 15000 <= Re-L <= 30000.