Abstract: Due to the frequent lost circulation of drilling fluids while running casing for cementing at the fourth and fifth sections in ultra-deep natural gas wells in the tectonic belt of Kuqa Piedmont in Tarim Basin, the actual shear rates of annular drilling fluid while running casing were analyzed. On the basis of the Herschel-Bulkley model, the test data collected at full shear rates (1.70–1021.40 s^–1) and low shear rates (1.70–340.50 s^–1) were used to fit the corresponding rheological parameters of the oil-base drilling fluid respectively, and to analyze the surge pressure difference under the two scenarios. The research found out that in Kuqa Piedmont, the actual shear rates during casing running were far less than the highest value (1021.40 s^–1) of regular full shear rates, and the rheological parameters varied greatly with the change of temperature and pressure at full and low shear rates. Moreover, the casing surge pressure at full shear rates was lower than that at low shear rates, and with the increase in well depth, the surge pressure difference between them was greater. The results showed that for natural gas wells with narrow safety density windows, the range of actual low shear rates of drilling fluid should be taken as the basis of fitting corresponding rheological parameters, and based on which, proper casing running speed should be designed to control the lost circulation according to the allowable surge pressure in the process.