Abstract: Provided that the flow of hole-bottom slurry(HBS)is a power-law fluid and flowing stably in concentric annulus space during the towback of horizontal directional drilling(HDD)operation and taking the influence of axial motion of inner pipe on flow field distribution into account, the analytic calculation method(ACM) about slurry pressure gradient and slurry shear stress at pipe outer surface are deduced based on the Navier-Stokes equation. Based on the HDD engineering examples, the data are analyzed by this method, and compared with the results of Baroid, SPE and Polak empirical formula, and ACM results are closest to the measured values. The slurry drag resistance is important part of the towback load and thus cannot be ignored in calculation. The expanding diameter ratio is the most sensitive factor in slurry pressure gradient and slurry shear stress at pipe outer surface in calculation.