Abstract: Surge pressure caused by pipe moving through the wellbore full of drilling .fluid is the main factor affecting wellbore stability, on which the additional mud density isdesigned. Thus if is desired to predict the surge pressure with high accuracy. While this is closely, related with the selection of mud rheological models. Yield-Pseudoplastic model which has three parameters and is.famous for its higher accuracy has been used to describe the mud rheological properties in recent years. Based on this model the paper presents a new theoretical model for calculating surge pressure caused by mud viscosity during tripping or casing in a concentric annulus of directional wells under steady laminar situation. For convenience, the paper plots the distribution of the coefficients of surge pressure for different conditions. An example is given. These results provide the basis for controlling pressure surges and tripping velocity.