A recording device for injection device includes a housing, a viewing window, a photographic module, a light splitting element and a signal transmission device. The housing is removably mounted on a dose scale window of an injection device. The viewing window is formed on a surface of the housing, and a viewing path is formed from the viewing window to the dose scale window. The photographic module is located on the housing, and provided with an image-capturing optical axis which is perpendicular to the viewing path. The light splitting element is located at an intersection of the viewing path and the image-capturing optical axis for guiding information located in the dose scale window to the photographic module and the viewing window, respectively. The signal transmission device is electrically connected to the photographic module for transmitting signals of the photographic module to an external device.