Based on the VOF model，CFD simulations were carried out for wall shear stress in gas-liquid vertical and inclined upward slug flow.The simulation results showed that the thickness of the falling film in the vertical upward slug flow was always smaller than that in the inclined upward slug flow.In the vertical tubes，wall shear stress increased gradually from Taylor bubble nose to the falling liquid film until it became stable in the developed falling liquid film，but in the wake zone the wall shear stress appeared irregular.In the inclined tubes，the tip of the Taylor bubble nose was above the centerline of the tube and the smaller the angle，the greater the thickness of the falling liquid film.Meanwhile，for the wall shear stress of slug flow in an inclined tube，when FrTB was small，the shear stress profile of falling liquid film on the top wall fluctuated significantly，but it was smooth on the bottom wall.With increasing FrTB， the difference between the shear stress profile on the top wall and the shear stress profile on the bottom wall gradually disappeared.