On top of our air condition was a drainage of the whole building, is it harmful to drop water on top of it?

Drain water contains grit and dirt so it will slowly degrade your a/c metal casing. Over time you will increase corrosion due to the action of the dirt which can become abraisive. You see the fall of water by gravity will act like a pounding hammer on your a/c and the dirt will slowly blast the paint and expose the metal. Your a/c compressor will be at risk of damage by water corrosion. Try to deflect the water by instally a shield above the ac unit. Talk to your building manager, such a drain is a building code violation.