How can I go Xian North train station to South train station by subway?

There is no direct subway service between these two places and the South Station is really far from the dowtown. Anyway, here is my suggestion:
take subway line to Fengxiyuan Station. Take exit D and walk for around 100m to Fengxi Lukou bus stop to transfer to bus no. 918 to the South Station in 50-60min.