The low drilling efficiency in medium hard rock construction for coal bed gas hole, mine vent, cable openings and holes and water source well； high drilling cost in water shortage area, water encounter collapsing and diameter shrinkage were all overcome by using RD20Ⅱ drill and air downhole hammer drilling technology. Based on the introduction of structure characteristics of RD20Ⅱ drill and QL series of downhole hammer, the paper summed up some problems in air downhole hammer drilling with solving thoughts and measures.