Abstract: In this paper, it analyzed the influence factors on gas pressure decline curve such as the length of manometry gas chamber, coal seam gas permeability coefficient and gas species injected into manometry gas chamber. The field test showed that the shorter the length of manometry gas chamber, the faster the gas pressure curve decline, and the shorter time required for the curve stable; The smaller the coal seam gas permeability coefficient, the slower the gas pressure curve decline, then the longer time required for gas pressure balance; The gas pressure decline rate of injecting CO₂ was greater than the rate of injecting N₂ in manometry gas chamber. Theoretical basis could be provided for the field pressure measurement through the analysis of each influence factor, thus the work efficiency was improved.