The invention relates to a tray (well) diameter detector, in particular to an aperture measuring device which is quite suitable for detecting pore-forming and tray-forming quality of a branch pile, has simple and clear principle, economical efficiency, practicability, obvious superiority in practicability and easy popularization and application, can provide technical parameters of well wall incline, well diameter, tray diameter and the like and effective technical support for the construction quality of the branch pile, and has favorable economical benefits and social benefits. The tray (well) diameter detector comprises an apparatus outer seat, an arm-drawing hydraulic cylinder, an inclining angle sensor, a diameter measuring arm, a pull rod, a diameter measuring tension spring, an apparatus lifting rope, an information output line and an oil supply pipe, wherein the diameter measuring arm is in close contact with a pore wall through the diameter measuring tension spring and the pull rod; the inclining angle connected on the diameter measuring reflects the changes of the diameter of the pore wall through the changes of an inclining angle; and the oil supply pipe supplies oil for the arm-drawing hydraulic cylinder so as to draw back the diameter measuring arm to be matched with the rotary angle of the detector, thus realizing carrying out up and down repeated measurement on the cavity of the tray from multi angles and detecting the tray diameter and shape of the branch pile.