A multimedia signal processing device is used between a multimedia signal source and a down-stream multimedia device. The multimedia signal processing device includes a communication unit, a multimedia decoding unit and a judging unit. The communication unit receives a multimedia signal from the multimedia signal source. The multimedia decoding unit decodes the multimedia signal and receives designated device information from the down- stream multimedia device, The judging unit performs an identification procedure according to a set of data stored in a database and the designated device information of the down-stream multimedia device to obtain a judged result to be stored in the multimedia signal processing device and have the multimedia signal processing device perform an action according to the judged result. A related authorization method involving the steps is performed by the multimedia signal processing device.