The invention discloses a regulating mechanism of an automatic capping device of a filling machine, comprising a regulating device. The regulating device comprises a transversally arranged power-driven shaft; the power-driven shaft is provided with two shifting blocks capable of moving left and right along the power-driven shaft; the two shifting blocks are connected with two clamp guide rods arranged above; the power-driven shaft is in transmission connection with a vertically arranged driven shaft; the driven shaft is provided with shifting blocks capable of moving up and down along the driven shaft; and the shifting blocks are connected with clamp guide rods arranged below. In regulating, the power-driven shaft drives the shifting blocks to drive the two clamp guide rods arranged aboveto move left and right, and meanwhile, the power-driven shaft enables the driven shaft to drive the shifting blocks to enable the clamp guide rods arranged below to move up and down relative to othertwo clamp guide rods by a series of transmission, thereby achieving the goal of regulating the distance between barrel caps at a time so as to adapt the barrel caps with different diameters and improving the capping efficiency.