The invention relates to an automatic braking system for reel. The automatic braking system comprises the reel for winding a fishing line and is characterized in that the reel is hollow inside, the inner wall of the reel is provided with a magnet which is capable of rotating along with rotation of the reel to generate a fixed magnet field, a brake control coil is arranged in the reel and capable of generating induced current when the magnet rotates along with the reel, and a closed current circuit is formed at two ends of the brake control coil. The induced current flowing through the brake control coil enables the brake control coil to generate an induction magnetic field in interaction with the fixed magnetic field generated by the magnet, so that a braking force is applied to the reel.Compared with the prior art, the automatic braking system has advantages that by means of noncontact braking, a mechanical brake pad is not needed, wear loss is avoided, and line blasting in a castingprocess can be effectively prevented.