I seems to have found out the reason,maybe the voltage provide by the newest Macbook Air,or the old connecting line.
When I full charged the iPad,it can be connected,but if it is not full charged,it would fail.I think that maybe the new Air can not provide enough voltage,I don't know if it is because of the power provider or the new battery.For I haven't got an iPad with the new connecting line,I can not admit it.Maybe the line is the reason,perhaps the compatibility,for this situation is often happened in the old facilities.