The invention discloses a free forging method of a forge piece of rotary arms, which comprises the steps of: firstly, making a forming die and an upsetting punch; secondly, first forging: heating a steel ingot, tapping and pressing a clamp handle, and removing the clamp handle to obtain desirable blank material of the forge piece; thirdly, second forging: putting a rotary upsetting table on a movable work table, putting the forming die on the rotary upsetting table, putting the desirable blank material of the forge piece into the forming die, putting the upsetting punch at the upper end face of one large circle of the blank material of the forge piece, and forging; and horizontally moving the movable work table to achieve the aim of performing; fourthly, third forging: installing a flat anvil on the end face of the large circle of the blank material of the forge piece, further widening, rolling to be a circle, and furnace filling; and fifthly, fourth forging: overturning the upper surface and the lower surface of the blank material of the forge piece, flatting the other large circle of the blank material of the forge piece, punching, rolling to be a circle, and finishing with manytimes to obtain a finished product of the forge piece of the rotary arms. The method has simple steps, and guarantees the forging quality.