This work reports a new means of preparing graphene tubes (GTs) without relying on chemical vapor deposition (CVD) and it's template-free. Surprisingly, we found that under the action of calcium oxide (CaO) and after 1500 °C heat treatment, a large amount of GTs grew on the surface of polyimide (PI). These nanotubes have a maximum diameter of about 600 nm and a length of up to millimeters, and some nanotubes even have a branching structure. We propose a simple, effective and green method which exhibits prospects for large-scale production of GTs using polymeric materials.