Using density functional theory (DFT) combined with nonequilibrium Green's function method (NEGF), the electronic properties of undoped, B/P, B/As, and B/Sb co-doping (8, 0) single-walled carbon nanotubes (SWCNTs) are investigated in the presence of external bias. All of the doping process will reduce the band gap of doping carbon nanotubes (CNTs) until it becomes a conductor. We find that undoped and B/As-SWCNTs have analogous I-V curve. And the negative differential resistance (NDR) effect will occur due to the localized electron or spread effect of electron throughout the molecule at the range of 0-0.3 V. When there is negative bias, negative differential resistance (NDR) effect occurs in the device of B/P-SWCNTs. But for the device of B/P-SWCNTs, the current increases slower from 0 to 0.3 V than that at the same positive bias. The electronic transport properties are strongly dependent on the presence of external bias. The results of our research will provide theoretical guidance for the application of carbon nanotubes in molecular devices (MDs).