An automatic tensioning system for strengthening a beam, a slab and a column by a pre-stressed FRP plate comprises a tensioning end anchor (1), a fixed end anchor (10), and a tensioning bracket (3) connected to the tensioning end anchor (1). A centre-hole jack (16) is provided in the middle of the tensioning bracket (3), a threaded rod (8) passes through the tensioning bracket (3) and is then connected to the centre-hole jack (16), and an upper toothed nut (13) and a lower toothed nut (19) respectively driven by a driving mechanism are provided on two sides of the tensioning bracket (3) on the threaded rod (8). A binary clip-type fixture (4) is further comprised, an upper surface thereof is provided with a cylinder (6), and a sleeve (7) nested outside the cylinder (6) is connected to the threaded rod (8).