If I arrive at 23:30 at Nanjing NKG, how can I get to zhenjiang

You need a taxi to Nanjing or Nanjing South Railway Station and then take a normal ride or high speed ride to Zhenjiang. There are normal rides from Nanjing Station from 00:03 to 04:38am in the early morning and the first high speed ride goes from Nanjing South Station at 05:36am.