A construction method of a hydraulic flushing and presplit blasting combined permeability-increasing extraction 
system in 
drill holes includes the steps of (1), forming the 
drill holes, namely forming a plurality of blasting 
drill holes and a plurality of flushing drill holes in an extraction roadway of a 
coal seam floor towards a 
coal seam direction in a staggered manner, wherein a distance between each blasting 
drill hole and the adjacent flushing 
drill hole ranges from 3 meters to 6 meters; (2), blasting, namely mounting a steel 
pipe at an orifice of each blasting 
drill hole, filling a gap between the steel 
pipe and the corresponding blasting drill hole with 
cement mortar hermetically, filling one end of a PVC (
polyvinyl chloride) 
pipe with 
emulsion explosive after the 
cement mortar is solidified, mounting a primer 
detonator closely to the 
emulsion explosive, filling a position, close to the primer 
detonator, with 
emulsion explosive, filling circularly sequentially and filling a position, close to the last primer 
detonator, with 
stemming, wherein a distance between each two adjacent primer detonators is one meter. According to the construction method, 
coal seam gas permeability is improved effectively, gas extraction effects are enhanced, extraction time is shortened effectively and potential safety hazards of gas are reduced maximally.