A computing system includes a central processing unit (CPU), a first direct memory access (DMA) controller, a first bus, a memory module inserted in a first dual inline memory module (DIMM) slot, and a storage device installed on a second DIMM slot. The storage device includes a storage module, a second bus, a memory unit, an interface control unit, and a second DMA controller. The CPU sets up the first and the second DMA controllers to perform data transmission. The first DMA controller controls a first data transmission between the memory module and the memory unit through the first and the second buses, and the second DMA controller controls a second data transmission between the memory unit and the storage module, with the interface control unit, through the second bus. The disclosure further provides a data transmission method of the computing system.