An expansion interface module having a signal gain circuit is disposed on a case panel of a computer and electrically connected to a main board which is disposed within the computer. The expansion interface module includes a circuit board, a connecting port, a signal gain circuit and a plurality of connectors. The connecting port, the signal gain circuit and the connectors are located at the circuit board and electrically connected thereto. The main board delivers signal to the circuit board through a cable which is connected to the connecting port located on the circuit board, the signal gain circuit amplifies and enhances the signal, and then the connectors output the amplified and enhanced signal to external devices, thus, the signal will not be attenuated, damaged or lost during over transmission distance.