The corridor resource for East China transmission lines is very tight. In order to make full use of the resources of overhead transmission line corridor，the same-tower ultra high voltage direct current（UHVDC）and alternative current（AC） hybrid transmission lines will be used in East China. According to the main circuit parameters of±800 kV UHVDC project from Ningdong to Shaoxing and the data of same-tower high voltage DC/AC hybrid transmission lines in East China， a model for simulation of AC/DC transmission lines is built with EMTDC software. The influence caused by such factors is calculated: induction in HVDC lines by voltages and currents in AC line， different DC operation modes and same-tower location of high voltage DC/AC hybrid lines. The AC frequency component distribution along the UHVDC line is given. The results after study show that AC induced component on DC line is rather small compared to rated voltage and current of UHVDC，so it has little influence on parameter selection of DC filter， smoothing reactor and converter valve， but it does have some influence on DC bias current of converter transformer and not good for the latter's operation. Finally， according to capability to withstand DC bias current of transformer， the permissible length of same-tower hybrid lines is given. The measure to suppress DC bias current of converter transformer is also put forward.