An expandable decoding circuit includes a latch unit, a latch result selecting unit, and at least one decoding circuit. The latch unit latches raw data and outputs the latch values and the latch inverse-values of the raw data. The latch result selecting unit composes the latch values and the latch inverse-values according to the target decoding value of the decoding unit to generate a pre-decoding value. The latch result selecting unit outputs the pre-decoding value to the corresponding decoding unit. The decoding circuit determines whether a decoding signal is outputted or not according to the pre-decoding value. Thereby, when a new function needs to be added to the deciding circuit, the present invention does not change the original decoding circuit and implements the decoding unit for the new function.