An adjustable heating mantle for tool holder includes a mantle body, a positioning plate, an adjusting unit, a lid and a lever rod. The positioning plate is mounted on the top of the mantle body and has an assembling recess. The adjusting unit is mounted in the assembling recess and has two magnetic shielding boards, a sliding ring and two connecting rods. A lid covers a top face of the positioning plate and is mounted on the top of mantle body. A lever rod is inserted through the lid and mounted on one of the magnetic shielding boards. The two magnetic shielding boards are capable of being rotated synchronously by moving the lever rod for shielding magnetic force and fitting cutting tools of different diameters.