The spectral response of 3mm×3mm position sensitive detector (PSD) covering a wide spectral range from 320nm to 1150nm. The beam deflection method was employed to determine the temporal response and position sensitive properties of the PSD. As an application of the PSD, we developed a horizontal atomic force microscope(AFM), introduced the basic concept and controlling system of the AFM, and presented some sample images scanned by the AFM. The results show that the horizontal AFM is an effcient system for image acquisition, which has an excellent repeatability, reliable stability and ideal image contrast. The maximum scan range is up to 5μm×5μm, and the resolution of the AFM is at the nanometer scale. It is obvious that the new PSD is of outstanding characteristics of spectral response, temporal response and position sensitive property.