A series-connected fan unit includes a first fan frame, a second fan frame, a first motor, a second motor and two impellers. The first fan frame has a first base and a first positioning member coupled with the first base. The second fan frame is connected to the first fan frame in series and has a second base and a second positioning member coupled with the second base. The first motor is disposed in the first base and has a first circuit board coupled with the first positioning member. The second motor is disposed in the second base and has a second circuit board coupled with the second positioning member. The two impellers are respectively coupled with the first motor and the second motor, and an insulation area is formed between the first circuit board and the second circuit board.