The invention relates to an automatic tensioning device and method for a winching rope. The automatic tensioning device for the winching rope comprises slide ways (3) which are fixed on an I-shaped front supporting leg (4-1) and an I-shaped rear supporting leg (4-2) and a sliding tensioning device (1) which is erected on the slide ways (3); and the sliding tensioning device (1) is connected with the I-shaped rear supporting leg (4-2) through a constant-force spring (2). Through feedback control, a winching tail rope is tensioned automatically to replace manual traction, the automatic tensioning device can be widely applied to mainstream winching machines, the safety and reliability of construction operation are improved, the human input is reduced, continuous tensioning force is guaranteed, and construction safety is improved.