How To Unclog Ac Drain?

Take your water hose and back flush it from the outside drain, where the ac drain comes out of the house run water from the hose through the pipe for about 5 seconds at the time, let the water drain out and you should start seeing gunk coming out of the drain pipe.

You can also loosen the fitting on the drain pipe inside where the A/C handler is and pour Clorox bleach down it. Then re-attach the drain pipe fitting.
The bleach will break down all the gunk in the pipe and push it on through the drain to the out side.