Abstract: The performance of coal-mine gas extraction borehole sealing material plays an important role in improving the quality of sealing in gas extraction boreholes and the efficiency of gas extraction. To address the issue of the ineffective expansion of traditional cement-based sealing materials in the initial stage of hydration, microcapsule technology was proposed to delay the expansion time of sealing materials by combining theoretical research, physical experiment. The experiment determined that delayed-expansion microcapsules were prepared by phase separation with ethyl cellulose (EC) as the capsule wall material and montmorillonite (MMT) as the core material. Moreover, according to the single factor experiment and response surface method (RSM), the interactions of various factors were determined. Experiment results showed that the significance order of the influencing factors was: stirring speed> ratio of core to wall> dosage of polyethylene (PE). Then, the optimal parameters for the preparation of microcapsules based on design-expert software were acquired: the ratio of core to wall was 0.8, the dosage of PE was 1.5%, the stirring rate was 400 r/min. Finally, the results of self-designed simulation experiment on the borehole sealing showed that delayed expansion sealing material has better sealing performance than ordinary expansion material, and its effective sealing time can be increased by about 60%.