A paper roll fixing device of a printer includes a first supporting base, a second supporting base, a first fixing member and a second fixing member. The first fixing member comprises a rotatable first fixing slab. The second fixing member comprises a rotatable second fixing slab. When the first fixing slab and the first fixing slab are pushed by a paper roll, the first fixing slab and the first fixing slab are rotated to the position without stopping the paper roll from entering or leaving the space between the first supporting base and the second supporting base. As a consequently, the paper roll can be smoothly introduced into or removed from the paper roll fixing device. By the paper roll fixing device, the paper roll can be introduced into or removed from the paper roll fixing device with a single hand or in a single step.