The invention provides a structural displacement monitoring system, wherein, a light transmitting device is used for arranging on the structure which is monitored. The light transmitting device, towards a light projective plane of the light receiving device, is setup for transmitting light on the light projective plane to form an optical spot. The light projective plane is disposed as a angle alpha from the light. A displacement measuring device of the optical spot, towards the light projective plane,is setup for measuring the optical spot displacement Z on the light projective plane for a short interval, and the displacement measuring device of the optical spot is connected with the signal of a structural displacement calculation device, which is used for transmitting the optical spot displacement Z to the displacement measuring device of the optical spot. The structural displacement calculation device is used for calculating the structural displacement *=z*Sina of the structure which is monitored in the direction of perpendicular to the light in a time interval. The invention also provides a relative monitoring method. The structural displacement monitoring system has the advantages of saving both time and labor in monitoring structural displacement, low cost, high precision, accurate and stable monitoring, being capable of real-time recording the change of structural dynamic displacement and being suitable for large-scale application.