To detect and control the nanometer distance between the sample and the fiber tip is part of the key technology of NSOM system. Here a pizeo sensor with DDS is discussed. One of the sensor′s electrodes is separated into two like parts, one is for exciting the pizeo to shake and the other is for detecting the shake. The fiber tip is fixed on the pizeo and the pizeo is driven by a DDS. When the fiber tip is far from the sample surface, an exact resonant frequency of the fiber tip is obtained with the error of 0.006 Hz after frequency scanning, and when the tip is within the near-field of the sample, the resonant frequency of the fiber tip departures the resonant frequency in far-field, and the amplitude of the fiber tip obviously decreases because of the shear force between the fiber tip and the sample, and in this way the near-field distance is detected. The combination of high precision frequency source DDS and the pizeo sensor provides a higher sensitivity and stability of near-field detection.