Abstract: An expansion structure of memory module slots is provided. A circuit switch board and a substrate are disposed on a motherboard, wherein a plurality of first memory module slots is disposed on the motherboard, at least one second memory module slot is disposed on one side of the substrate, and a plurality of third memory module slots is disposed on the other side of the substrate. The third memory module slots are electrically connected to the second memory module slot, and two ends of the circuit switch board are plugged into one of the first memory module slots and the second memory module slot. When memory modules are plugged in the third memory module slots, the memory modules are electrically connected to the first memory module slots respectively through the circuit switch board and then transmit data and signals with the motherboard.