public var file:FileReference = new FileReference();

public function doSave( event:Event ):void
{
        // txtField is the instance name of a text field with contents to save.
	var data:ByteArray = new ByteArray();
	data.writeMultiByte ( txtField.text, "utf-8" );
	file.save( data, "myfile.txt" );
}

Adobe AIR - Save text to local text file

file, actionscript, text

ActionScript 3