Abstract: In order to study the variation law of pressure in the directional long borehole, the pressure distribution model in the directional long borehole is deduced based on the mass and momentum analysis method and the similarity principle combined with the gas flow test of the PVC pipeline. The results show that: the velocity of the mainstream in the borehole is exponentially distributed along the length of the pipe, and the momentum exchange coefficient k is linearly related to the relative kinetic energy difference in the borehole; momentum exchange plays a decisive role in the total pressure loss, which cannot be reflected by the Bernoulli equation; the negative pressure in the borehole is proportional to the negative pressure at the orifice, and the loss of negative pressure in the hole has nothing to do with the negative pressure at the orifice; the negative pressure in the directional long drilling hole has an exponential relationship with the drilling depth; the presence of one hole diameter minimizes the negative pressure loss in 100 meters in long holes.