The utility model discloses an expansion joint of a three-phase AC energisation contact rail, which comprises two guide rails (1), two guide rail racks (2) and two guide rail trough plates (3). The cross section of the two guide rail racks (2) is T-shaped. The two racks (2) are symmetrically arranged in the two guide rail trough plates (3) by the pins (5) fixedly mounted on the back side of the rack bottom I (4). The outer ends of the two guide rail racks (2) are equipped with connection plates (9). The cross section of the two guide rails (1) is also T-shaped. The two guide rails (1) are inserted at the two sides of the rail heads I (6) of the two guide rail racks (2). The connection plates (9) are inserted and fixed inside the mould cavities at the ends of two adjacent contact rails, so that the openings of the two guide rail trough plates (3) are turned to be outward. According to the utility model, the influence of the temperature change of the environment on the longitudinal length of the contact rails during the running process is compensated in a manner of expanding with heat and contracting with cold, thereby ensuring the normal mechanical and electrical connection of the lines under different running environments and the running safety of the locomotives.