Is there any subway line I can take from the city to Wuhan Airport?

I am afraid not. But you may take an airport shuttle bus from the main railway station, Qingyang Passenger Transport Station, Hongji Passenger Transport Station, Fujiapo Passenger Transport Station, Xinrongcun Passenger Transport Station, etc. They shall directly take you to airport.

Thanks Bob for your advice. I notice that in the future line 2 will be connected to the airport. Do you think it is a bad idea to take the subway (line 2) until the last station and then take a taxi from there to the airport?

Well, I don't think it is wise to take line 2 to the terminal and then taxi to airport. The terminal - Jinyintan is still around 22km from the airport and it costs CNY60-70 by taxi. The best way is to use shuttle bus.

By "main railway" station, do you mean the WH Railway station?

Yes, you got it.