Abstract: Borehole sealing technology is a key technology for the successful development of water injection in coal seam. The high pressure required by high pressure water injection in coal seam depends on whether the sealing effect is good or not. To solve the problem of sealing failure of drilling caused by unreasonable sealing length, theoretical analysis and field application of sealing length are carried out. Two conditions for effective sealing of capsule sealing device were proposed, and a theoretical model of capsule sealing mechanics was established. The research results show that the width of the plastic zone of 202 return air lane in Gaojiapu Coal Mine is 8.95 m, the length of the capsule is 8.1 m at least, the sealing length should be greater than 8.95 m, and the sealing length of the hole is 10 m, which meets the requirements of drilling sealing.