Instructions
Divert your horse's attention from what frightens him. This is particularly effective if you see something you know has spooked your horse in the past before he sees it.
Walk your horse, leading him from side to side, or maybe even having him take a few steps to the rear. Giving him something else to think about will help you to calm down a spooked horse.
Reward him with reassuring words and by petting him when he begins to calm down. This will help him to associate a positive with controlling his fear.
Know your horse's fears and work to de-sensitize him slowly. For instance, if he has spooked in the past near a certain bridge, take him to that area but don't force him to go very close. Continue this training, getting a little closer each time and you may find eventually he'll be able to cross the bridge without spooking at all.
Be aware of your own body language. If your horse has spooked and you are able to maintain a calm demeanor, your horse will likely calm down much more quickly.
How to Calm Down a Spooked Horse
Horses have a deeply ingrained "fight or flight" instinct, so many of them spook easily. Something as simple as a grocery bag blowing into your path may be enough to get your horse to take off running and you're in for quite a ride unless you know how to calm down a spooked horse.