The invention provides a hydraulic continuously adjustable digital display type tensioning device and a tensioning method. Adjustment of the large-stroke tensioning amount of a steel wire rope can be achieved through a small-stroke hydraulic cylinder. By means of the combination between rough adjustment, conducted by the hydraulic cylinder, of the tension value and fine adjustment of an adjustable stud bolt connection device, the tension value of the steel wire rope can be accurately adjusted to be equal to a preset tension value. According to the hydraulic continuously adjustable digital display type tensioning device, the structure is novel and simple, and installation and adjustment are convenient and accurate. The tensioning method is safe and reliable, the efficiency is high, the cost is low, the later-period operation and maintenance cost is also low, and popularization and application are quite facilitated.