The invention discloses a processing precision property online measurement device and a processing precision property online measurement method for a super-heavy lathe. Laser spot images are acquired by using a high-speed linear array image sensor, clear spot images are acquired from a group of acquired spot image sequence by using image recovery technology, sub-pixel central points of the spot images are identified by using correlation of pixel gray between the image sequences, and position information on the surface of a rotary workpiece can be effectively acquired by trigonometric imaging path calculation so that the technical defect that the conventional optical measurement method and the conventional optical measurement device cannot effectively acquire the position information on the surface of the rotary workpiece is overcome. Based on the acquired position information on the surface of the rotary workpiece, the developed measurement device is arranged on the heavy lathe; and after finishing a debugging calibration process, the measurement device can realize online on-site measurement of the processing dimension of the rotary workpiece on the heavy lathe, and meanwhile, themeasurement system also has the function of positioning the processed workpiece.