A double rectangular comb slow-wave structure with a round beam channel is proposed as the slow-wave structure (SWS) circuit of a broad bandwidth traveling-wave tube at W-band. The "cold" characteristic of the SWS and the transmission of the input/output structure are simulated and optimized in this paper. The results show that this circuit has a very broad bandwidth, and that the impedance of the structure with round beam channel can be the same as that of sheet beam by modifying the distance between the double vanes and the radius of the beam channel. Moreover, the transmission of the whole tube is so good that the VSWR can be kept at below 2 in a very broad bandwidth after optimization. The output performance is investigated by both the big signal theory MTSS and the PIC code MAFIA, and they have the same results that in 10 GHz bandwidth the output power is over 40 W and the gain is above 29 dB under a driven power of 50 mW.