Eating dirt is a form of pica, or eating things that aren't food.  In some cases, pica is caused by a mineral deficiency in your dog's diet or a parasite infestation.  In other cases, however, eating dirt may simply be a sign of boredom. This is especially true in puppies and young dogs. A dog may also eat dirt to alleviate intestinal distress caused by eating something he shouldn't have. Were there any recent changes in the dog's diet, level of activity, or environment?  Does the dog show any other unusual symptoms or behaviors that might help explain the problem?  If you have multiple dogs, are they all eating dirt?  If your dog eats an unusual quantity of dirt, keep a close eye on him in case he did eat something he should not have and is experiencing signs of poisoning. If multiple dogs in a household are all eating dirt, it could be a sign of a deficiency in their diet. If multiple dogs are all eating dirt from the same location, however, it could be that there is something delicious in that dirt. Eating dirt may be a sign of anemia caused by dietary deficiencies or parasites. If the dog's gums are pale or jaundiced (yellow in color) take him to the vet right away for further diagnosis and treatment. Most commercial dog foods contain enough vitamins and minerals to keep a dog healthy, but some brands contain less minerals than others.  Check the label of your dog food and compare it to other brands to make sure it has similar amounts of minerals like iron and calcium. If your dog eats a raw or home-cooked diet, he may not be getting the nutrients he needs.  Talk to your vet about mineral supplements in that case. A dog may also eat dirt because he is hungry. If you have cut back on your dog's food because he is overweight, consider a lower-calorie food that keeps him feeling full instead of less of his normal food. If you can't figure out the cause of your dog's dirt-eating, or suspect it may be a sign of a more serious problem, bring the dog in for a check-up. During an exam, the vet can check the dog's...

TL;DR in Chinese:
了解狗吃土的部分原因。 想想狗狗是从何时开始吃土的。 检查狗狗的牙龈。 看狗狗的饮食是否出了问题。 咨询兽医。