I'll arrive at Guilin airport at 19 00 pm.Want to go to Yangshuo direct. Which is the best way ?

Yes, you need to take a bus to Downtown area first.
If you take the bus at GL Bus Station, it may be too late to catch the bus. So you are suggested to take a taxi for GL Railway Station from the place you get off the airport shuttle bus, there are buses to Yangshuo in service still at that time. In the airport, there are taxis available to Yangshuo, too. And the taxi fare is about CNY 250 – 300.