It can increase line corridor utilization when AC transmission lines are set parallel with UHVDC transmission lines. At the same time, the AC transmission line which is parallel with UHVDC line may cause electromagnetic influence upon UHVDC transmission line. The power frequency AC component will generate along the UHVDC transmission line. The power frequency AC current will be transformed into DC current under the action of current converter. When flowing into converter transformer, the DC current can cause transformer magnetic bias. Based upon EMTDC software, the simulation model of UHVAC/UHVDC transmission lines is set up. The electromagnetic influence of AC transmission line on UHVDC transmission line in parallel is studied. The factors such as parallel erection length, adjacent distance, conductor transposition and single phase grounding fault that affect the electromagnetic induction influence upon UHVDC transmission line are analyzed. The power frequency induced voltage, induced current and DC bias current in UHVDC lines which are paralleled with UHVAC/EHVAC transmission line and UHVAC single or double-circuit transmission line are compared and analyzed.