My serpintine belt keep coming off i replaced pulley and tensione

Typically a belt will come off if all the pulleys are not perfectly aligned.
If you recently replaced a power steering pump and the pulley was not properly seated all the way down where the pulley front is flush with the metal shaft from the pump then you will shred the belts and they will come off. If you ONLY swapped the tensioner and idler pulley you may have installed the tensioner so it isn't parallel to the other pulleys but that would be very rare because it would have been difficult to get the belt on if it was not properly installed. There also could be a damaged plastic pulley that could be "cutting" into the belt which would also cause the belt to slip off.