Abstract: A multistage module expansion system and multistage module communication method, applicable to a set-top box, are introduced. The system includes a master module, at least a preceding expansion module, and at least a succeeding expansion module. The master module generates and sends a control instruction to the preceding expansion module and the succeeding expansion module. The preceding expansion module and the succeeding expansion module each determine whether the control instruction is of a type executable by the preceding expansion module and the succeeding expansion module, respectively. If the determination is affirmative, the preceding expansion module creates and sends a preceding data packet to the master module, and the succeeding expansion module creates and sends a succeeding data packet to the preceding expansion module, such that the preceding expansion module sends the succeeding data packet to the master module.