The invention discloses a precision hole aperture detector. The detector comprises a shell and a dial indicator; an axle hole is formed in the shell, a measuring bar of the dial indicator extends intothe axle hole from an opening; a pair of symmetric radial through holes are axially arranged at the positioning, close to the axle hole closed end, in the shell; annular limiting blocks are arrangedat the connecting parts of the axle hole and the radial through holes in the axle hole; a floating measuring head penetrates each radial through hole, and each floating measuring head comprises an abutting part capable of extending into the axle hole and a measuring part extending to the external of the radial through hole; the floating measuring head can move in the radial through hole; when axially sliding, a wedge-shaped sliding block upwards pushes a measuring rod or falls to the bottom of the axle hole by keeping away from the measuring rod; when the floating measuring head moves to the axle hole direction, the abutting part applies force to the wedge-shaped sliding block, so that the wedge-shaped sliding block slides to the measuring rod direction. The precision hole aperture detector disclosed by the invention can reduce the difficulty of measuring the precise hole aperture after being used, prevent the hole wall of an inner hole from being scratched, and the detection efficiency is improved.