The present application discloses a computing system and a memory sharing method for a computing system. The computing system includes a first host, a second host, a first memory extension device and a second memory extension device. The first memory extension device is coupled to the first host. The second memory extension device is coupled to the second host and the first memory extension device. The first host accesses a memory space of a memory of the second host through the first memory extension device and the second memory extension device according to a first physical address of the first host.