An assembly system warning device adapted to be disposed on a child safety seat, the assembly system warning device comprising: a first assembly member and a second assembly member configured to assemble the child safety seat into a vehicle, each of the first assembly member and the second assembly member having an assembly position and an idle position with respect to the child safety seat; characterized by the assembly system warning device further comprising: a main circuit having a first end and a second end and being connected in series with a power supply and an alarm device; a first circuit; a second circuit; a first switch having a first end electrically connected to the first end of the main circuit and a second end selectively and electrically connected between the first circuit and the second circuit; and a second switch having a first end electrically connected to the second end of the main circuit and a second end selectively and electrically connected between the first circuit and the second circuit; wherein when the first assembly member is located at the assembly position and the second assembly member is located at the idle position, the main circuit and the second circuit are connected in series to form a closed circuit; when the second assembly member is located at the assembly position and the first assembly member is located at the idle position, the main circuit and the first circuit are connected in series to form a closed circuit. a first assembly member and a second assembly member configured to assemble the child safety seat into a vehicle, each of the first assembly member and the second assembly member having an assembly position and an idle position with respect to the child safety seat; characterized by the assembly system warning device further comprising: a main circuit having a first end and a second end and being connected in series with a power supply and an alarm device; a first circuit; a second circuit; a first switch having a first end electrically connected to the first end of the main circuit and a second end selectively and electrically connected between the first circuit and the second circuit; and a second switch having a first end electrically connected to the second end of the main circuit and a second end selectively and electrically connected between the first circuit and the second circuit; wherein when the first assembly member is located at the assembly position and the second assembly member is located at the idle position, the main circuit and the second circuit are connected in series to form a closed circuit; when the second assembly member is located at the assembly position and the first assembly member is located at the idle position, the main circuit and the first circuit are connected in series to form a closed circuit. a main circuit having a first end and a second end and being connected in series with a power supply and an alarm device; a first circuit; a second circuit; a first switch having a first end electrically connected to the first end of the main circuit and a second end selectively and electrically connected between the first circuit and the second circuit; and a second switch having a first end electrically connected to the second end of the main circuit and a second end selectively and electrically connected between the first circuit and the second circuit; wherein when the first assembly member is located at the assembly position and the second assembly member is located at the idle position, the main circuit and the second circuit are connected in series to form a closed circuit; when the second assembly member is located at the assembly position and the first assembly member is located at the idle position, the main circuit and the first circuit are connected in series to form a closed circuit.