This paper presents the exploration process of expanding cutoff boundary of Tongbai Anpeng bicarbonate mine, and put forward the basic principles of exploring cutoff boundary of mirabilite mine and applied theory, technology and methods of exploring cutoff boundary. This theory, technology, methods are attested practical, the results of expanding cutoff boundary are basically consistent with the development, and thus provided the basis for bicarbonate production and continuing expansion, reducing unnecessary waste and costs.