Provided is a cooling system for a dry-type air-core reactor. An air-core reactor (4) with natural-air cooling of the winding (1a-1c) includes at least a first open space (3) to let air flow through the winding (1a-1c) in parallel with an axis of symmetry (A) of the reactor (4) and at least a second open space (2a, 2b) crossing the first open space (3) to let air flow through the winding (1a-1c) angular to the axis of symmetry (A). A ventilation unit (6, 7) to produce a forced-air flow (15, 17, 18, 21) is arranged in such a way to the air-core reactor (4) that a first part (15) of the forced-air flow enters one of the first (3) or second (2a, 2b) open spaces and at least one guiding element (14a) is arranged with respect to the crossing of the first (3) and the second open spaces (2a, 2b)in such a way that the first part (15) of the forced-air flow leaves and a second part (17) of the forced-air flow enters the one of the first (3) or second (2a, 2b) open spaces.