How do dogs spit up yellow frothy mucus?

If the yellow mucus your dog is vomiting is gastric juice, is not accompanied by other adverse symptoms and is in good spirits, fast your dog first, prepare some warm water for your dog and feed him some probiotics to regulate his gut. However, if the dog is vomiting along with other symptoms, such as poor spirits, persistent vomiting and fever. Take your dog to the vet for examination and treatment.