What 's trhe reason of water leakage in the duct when the slope, operation of ac are allright?

Well, something isn't right. The act of cooling the air removes moisture.

At the bottom of the evaporator in the furnace is a drain pan. This is connected to a drain. The drain may have a condensate pump on it. This drain may be plugged.

High moisture areas will also cause the metal duct to sweat which can look like water coming out of the duct.