Disclosed are a ribbon cartridge, a thermal transfer printer, and a method for installing the ribbon cartridge. The ribbon cartridge (2) comprises a bracket (23), a supply cylinder (21) arranged on the bracket and used for accommodating an unused ribbon, and a winding cylinder (22) arranged on the bracket and used for accommodating a used ribbon, where the supply cylinder and the winding cylinder are placed in parallel, and have an initial state in which the supply cylinder and the winding cylinder are close to each other and an open state in which the supply cylinder and the winding cylinder are away from each other under the action of an external force. Compared with a ribbon cartridge whose supply cylinder and winding cylinder are arranged at a fixed interval in the prior art, the ribbon cartridge as disclosed is more compact and needs less installation space.