There are two main methods to whiten teeth: 1. ultrasonic scaling and dental surface polishing. 2. cold light whitening. Ultrasonic cleaning method is mainly for exogenous coloring of tooth surface, such as people who smoke, drink tea and brush teeth uncleanly for a long time, there will be tartar and melanin on teeth, then ultrasonic cleaning can remove these pigments and restore the original color of teeth, so as to achieve the effect of whitening teeth. Cold light whitening technology is mainly for endogenous tooth staining, such as ageing yellowing, natural yellowing of teeth, tetracycline, fluorosis and dead pulp teeth. Under a certain cold light source, the bleaching agent can be introduced into the dentin to neutralize the pigment in the dentin and provide a whitening effect. In addition, if the tooth after root canal treatment, the tooth color will also gradually darken, usually you can use the method of wrapping porcelain crowns or all-porcelain crowns, which can not only solve the problem of tooth aesthetics, but also have a protective effect on the crown to prevent tooth fracture.