There are always some blood come from my teeth, many times one day, why?

Possibly your gums: Assuming you mean bleeding from the gums surrounding your teeth it is most likely caused by inflammation which is most often caused by poor plaque control. Some medical conditions can contribute to this as well so the best plan is to get evaluated by a dentist and develop a treatment plan after reviewing all pertinent medical information.

Bleeding gums: You may have gingivitis or/and periodontal disease. Plaque is a sticky bio film that can be removed with a toothbrush, calculus is a harder and attaches itself to the tooth and root surface and needs to be removed professionally by a hygienist or dentist. See you dentist for an evaluation.

Gingivitis : Gingivitis and periodontitis are conditions where the gingival tissues remain unhealthy even though you brush and floss. Further treatment is needed to correct the disease process continuing in the gums. Once the gums are healthy, the bleeding will stop.