The utility model provides a gap measuring device. The gap measuring device comprises a measuring rod, a measuring cylinder, a dial gauge measuring point rack, a dial gauge rack and a dial gauge, the measuring rod penetrates through the measuring cylinder and slides relative to the measuring cylinder, a first end of the measuring cylinder is provided with the dial gauge rack, a second end is provided with a positioning portion, the dial gauge is arranged on the dial gauge rack, the dial gauge comprises a pressure-measuring head, the dial gauge measuring point rack is arranged on the measuring rod and relative to the pressure-measuring head, and one end of the measuring rod is also provided with a measuring head. The utility model also provides an axial gap measuring device of a pneumatic pump, including the above gap measuring device. According to the gap measuring device and the axial gap measuring device of the pneumatic pump, the axial gap can be directly read from readings of the dial gauge, the efficiency and accuracy of axial gap measurement are improved, and the maintenance processes are greatly and conveniently simplified.