Ultra High Voltage Direct Current (UHVDC) has a rapid development in China by means of its special large capacity, long distance, high voltage and so on. This paper carries out a series of research on AC non-characteristic harmonic currents of UHVDC system. Firstly, it theoretically analyzes the appearance of 5/7 times non-characteristic harmonic currents at the unbalanced converter transformer impedance and trigger angle of UHVDC. Then, according to a real domestic UHVDC operation parameter, a simulation module is established by PSCAD/EMTDC. Finally, aiming at two major cases that the unbalanced converter transformer impedance and trigger angle of UHVDC, it calculates the AC 5/7 times harmonic currents. The simulation results show that the unbalance converter transformer impedance has less influence on AC/DC power system while the lopsided trigger angle indeed has a relatively large influence that AC 5/7 times harmonic currents appear greater, exceeding the current harmonic content maximum standard of 1%.