A structure for playing MIDI messages including a main-control element, a buffer memory, a MIDI synthesizer, and a pre-processor is provided. The main-control element receives tone colors and the MIDI messages, and loads at least part of the tone colors to the buffer memory. The main-control element transmits the MIDI messages to the pre-processor. The pre-processor coupled to the main-control element and the buffer memory, is used for analyzing the MIDI messages. In addition, the pre-processor dynamically determines at least part of the tone colors to be saved in the buffer memory based on the capacity of the buffer memory.