The invention relates to the field of tensioning devices, and discloses an automatic tensioning device for an electric generator. The automatic tensioning device comprises an engine, the electric generator, a driving wheel, a driven wheel and a belt. The engine is provided with a first side wall and a regulating device. A rotating base is provided with a second side wall. The first side wall and the second side wall form a V-shaped groove. The regulating device comprises a piston cylinder and a piston. The piston is connected into the piston cylinder in a sliding mode. The piston is conical and abuts against the first side wall and the second side wall. A fixing ring coaxial with the electric generator is arranged on the periphery of the electric generator. A plurality of inflators are fixed to the inner wall of the fixing ring. Air plugs are connected into the corresponding inflators in a sliding mode, and compressed springs are arranged between the air plugs and the outer wall of the electric generator. All the inflators communicate with the piston cylinder through an air pipe. The automatic tensioning device for the electric generator is provided, and accordingly the electric generator belt can be tensioned automatically.