A motherboard testing device applied to a motherboard which includes two memory channels, and a CPU. Each of the two memory channels includes two memory slots. The motherboard testing device includes four memory modules received in the four memory slots, a switching chip, a microcontroller, and a testing module. The switching chip includes four input pins electrically connected to the four memory modules, four output pins electrically connected to the CPU, and a controlling pin electrically connected to the microcontroller. The microcontroller forms a plurality of combination modes of the memory slots by electrically combining the four memory slots, and controls the switching chip to electrically connect memory slots of each combination mode to the CPU. The testing module tests whether the CPU controls the memory modules received in the memory slots of each combination mode to work in proper working modes.