The invention discloses a method for sealing a large-dip-angle downward gas pressure determination drill hole. Through multiple plugging means, the phenomena of gas leakage and liquid leakage caused by that a downward hole is not tightly sealed can be avoided, the hole sealing quality can be improved, and the accuracy of determining gas pressure of a coal seam is guaranteed. The method comprises the following steps that firstly a large-dip-angle drill hole of which the diameter is 75mm is constructed towards a downward target coal seam from a tunnel; two discs are connected to the tail part of an antipriming pipe with bolts, and crude cotton yarns are clamped between the two discs so that a mop shape is formed; a pressure measuring pipe is conveyed into a specified drill hole position, and pebbles and coarse sand, with different particle diameters, are sequentially stuffed in the drill hole; finally the drill hole is filled with hole sealing slurry so that the drill hole is sealed. According to the method disclosed by the invention, the difficult problems that when polyurethane is used for blocking the slurry, the polyurethane is very fast to expand and is not easy to convey to a specified point and the hole sealing length is short are solved, and the difficult problems that the slurry blocking effect is not good, so that the slurry leaks into the hole bottom of the downward hole, and gas pressure measurement or extraction and exploitation of gas are influenced are also solved. The method disclosed by the invention has the advantages that the processes are simple, the reliability is high, the slurry blocking effect is good, the construction efficiency is high, the prices of the hole sealing materials are low, and the method is suitable for being widely used in mines.