The digestion instrument adopts a surround heating method to digest the sample under normal pressure, and uses wet digestion method to achieve fast and stable sample processing effect. In the digestion of insoluble samples, an electric negative analyzer can replace microwave and electric heating plates and is a sample pretreatment instrument for analytical instruments such as AA, ICP, ICP-MS. At the same time, the electric thermal digestion instrument (acid removal instrument) can be used for microwave digestion pretreatment and acid removal treatment.