Abstract: Trajectory design of the near-horizontal directional drilling technology in underground coal mine is one of key aspects to successfully implement the technology. The trajectory of hole in a nearly horizontal formation is designed generally by two-dimensional method, and this method usually leads to the problem that the deflecting section of the hole travels out of the inclined target formation.Therefor, the calculation methods of the drilled formation true thickness were studied in the paper, the method using the unit normal vector of formation was proposed, and computation table in Excel was developed to guide trajectory design of near-horizontal directional drilling. The reliability of the method is verified in oblique section construction of a nearly horizontal directional hole in the limestone of Taiyuan Formation in Pansidong Mine, which has certain guiding significance for similar engineering problems.