Abstract: A novel optoelectronic detecting method based on projective geometry is proposed to monitor the horizontal and vertical displacement of measured points of a structure such as dam and huge straight building. In the method, a transparent mask is taken as a substrate, a measurement grating is made on the substrate and the mask is fixed on the measured structure. The relative space relationship of the datum line and mask is represented by the beam from GRIN lens. The return beam with mask mode information is transferred by the receiving lens to the monitoring centre through a cable. The horizontal and vertical displacement of measured points of the structure is obtained with optoelectronic transform, A/D and signal analysis. Experiments show that the measurement accuracy of horizontal and vertical displacements can reach 0.005mm and 0.1mm respectively. The sensors at measured spots do not have any electronic element and they have good adaptability and anti-interference performance in the harsh environment.