The invention discloses a device for detecting a distance between a needlepoint of a tapping-mode atomic force microscopy and a sample. The device comprises a DDS signal generation circuit and a phase lock amplification circuit respectively connected with a PID controller of the prior atomic force microscopy, as well as an improved detector consisting of a double-arm quartz tuning fork and a probe. During information detection, the device adopts an excitation voltage signal as a driving mode of tuning fork vibration and then determines the distance between the needlepoint and the sample by detecting the current passing through the tuning fork. The distance detection adopts phase lock amplification and can obtain a sample appearance image and a phase image at the same time. Moreover, a detector designed by the invention has simpler structure, small volume and high reliability.