What line should we use to get from Chongqing expo center to the city center ?

Well, what do you mean the city center? Take Chongqing Railway Station as an example, take metro Expo Line to Lijia Station and transfer to metro line 6 to Hongqihegou Station, Exit 2. Then take bus no.112 (Outer Ring) to Caiyuanba Bus Stop. Afterwards, walk southwest around 150 meters to find the destination.