Abstract: Due to some characteristics of the coal seam such as the low pressure, easy to collapse and leakage, resulting in the irregular borehole in coal seam segments and serious hole enlargement, the displacement efficiency in the process of cementing is reduced, which affects the sealing effects of slurry in cementing. In this paper, through the use of CFD numerical simulation method, combined with Hegang CBM well logging data, various parameters of the different coal seam segments such as the length of the hole enlargement, hole enlargement ratio, deviation angle, the centralisation degree of casing, the slurry rheological parameters and the value of volume displacement efficiency under the replacement fluid state and cross-sectional displacement efficiency are simulated. The influence of various factors on the displacement efficiency is analyzed. The displacement efficiency will be improved with the length of the hole enlargement segment increasing. On the contrary it will be reduced with the hole enlargement ratio, the deviation angle and the eccentricity boosting. Bingham fluid and power law fluid both have the highest rheological parameter values of displacement efficiency. The plug flow displacement is recommended under replacement fluid states. The above research results have provided the technical guidance for the safe and high quality cementing of CBM wells.