A cutting-off structure of a packer includes a main body having one side provided with a strap-cutting device. The strap-cutting device has a cutter tube formed with an application section and a combination section, and the application section is provided with a first strap-cutting block and a second strap-cutting block and formed with a strap-guiding groove and a strap-cutting opening. The first strap-cutting block is disposed with a strap-cutting surface at one side opposite to the strap-cutting opening for threadably assembling a cutting unit thereon, which can be assembled with or disassembled from the strap-cutting surface. By so designing, the strap-cutting surface of a packer in the present invention enables a user to cut off packing straps quickly and accurately through the strap-cutting opening and the replaceable cutting unit, convenient in use and able to lower cost.