In this paper 2D numerical model is used to study the effect of depth of airway surface liquid (ASL) on the muco-ciliary transport. An immersed boundary-lattice Boltzmann method is used to solve the momentum equation. In this study mucus is considered as the viscoelastic fluid an Oldroyd-B model is used as the constitutive equation of it. Immerse boundary method is used to study the propulsive effect of the cilia and also the effects of mucus– periciliary layer interface. Our results show that mean mucus velocity becomes maximized when the PCL depth is equal to the standard value of it i.e. 6 μm. By increasing or decreasing the depth of PCL or increasing the depth of mucus layer, mean mucus velocity reduces. Our study also shows that mucus viscosity ratio can play an important role on the muco-ciliary clearance. It means that by increasing the Newtonian part of mucus viscosity or by decreasing elastic contribution of the mucu, mean mucus velocity increases significantly. So reducing mucus velocity results from changing ASL depth can be completely modified by increasing the Newtonian part of mucus viscosity.