A filling is painful, depending on the degree of tooth decay. The filling is a kind of filling treatment after the tooth decay, it needs to remove the decayed part, when removing the decayed part with the dental high-speed handpiece, the drilling and grinding of the tooth tissue will have a certain pressure on the pulp tissue, if the decay reaches the dentin layer, the dentin tubules will transmit the external hot, cold and mechanical stimulation to the pulp tissue, causing the pain reaction of the tooth nerve, the closer the decayed part is to the tooth nerve, the more obvious the pain. After the decayed part is removed, the filling treatment will be carried out, and the filling material will expand and put pressure on the nerve, which will cause short-term pain reaction, which may only be transient pain and will disappear gradually. If the tooth decay is shallow and limited to the enamel, the enamel is a layer of protective tooth tissue without sensory function, so there will not be any sensation during the drilling and grinding process, so the filling should be done before it is too late, and the pain will not be obvious if the tooth decay is shallow. If the tooth decay reaches the tooth nerve, there will be spontaneous pain and cold, hot, sour and sweet stimulation pain, and the pain will be very intense during the drilling process. The pain level of filling depends on whether the tooth decay is close to the nerve or not.