Abstract: Pressure loss of cement column after cementing often results in reduction of pressure exerted on the gas zones by the liquid cement slurry column. This pressure loss may cause underbalance of pressure to happen and hence gas channeling at the early stage of cementing. Accurate evaluation of the pressure loss is a key to the formulation of techniques regarding the extent of pressurization and WOC and to the success of well killing and prevention of gas channeling. Empirical formulas for calculating pressure loss of cement slurry and results obtained from apparatus presently in use for measuring pressure loss of cement slurry do not reflect the pattern of pressure loss of cement slurry in downhole conditions. To address these problems, an apparatus for evaluating the pressure loss of cement slurry at HTHP conditions was developed by scaling down the sizes of existed apparatus. With this new apparatus, experiments have been performed on the typical pressure loss behavior of cement slurries in vertical and horizontal wells. It was found that at time of initial gelling, the pressure exerted by a column of cement slurry with poor stability was possibly less than the pressure exerted by a column of water of the same height. When designing the value of pressure exerted at the annulus using empirical formulas, the designed pressure may not be able to balance the pressure inside the annulus, thereby resulting in gas channeling, or the designed pressure may be higher than necessary to fracture the formations. A new method was developed to segmentally calculate the pressure loss of a cement slurry. This new method was used in calculating the pressure loss of a high-pressure gas well in the Block Moxi-Gaoshiti in Sichuan Basin, providing helpful guide to the pressurization in annulus and WOC operations. The job quality of well cementing was certified and no gas channeling happened after well cementing.