A liquid-immersion cooling device includes a reservoir, a partition board, and a heat exchanger. The reservoir contains coolant to immerse the electronic device. The partition board and the heat exchanger are mounted within the reservoir. The partition board is arranged parallel to the electronic device. The heat exchanger and the electronic device are respectively arranged on opposite sides of the partition board within the reservoir. The partition board is configured to guide a flow of the coolant within the reservoir. When the coolant flows through the electronic device and carries away heat generated by the electronic device, the coolant carrying the heat is guided by the partition board to flow through the heat exchanger. After exchanging heat with the heat exchanger, the coolant is guided by the partition board to flow through the electronic device.