The square chisel is an important trench cutting tool for the underground diaphragm wall. This paper briefly introduces the structural design， construction method and application of a new square chisel. The integral frame of the new square chisel is welded with proper structure， and its size can be adjusted according to the needs of users. It has the advantages of good guidance， large water passage， high impact power per unit area and ease for fishing. After the successful development of the chisel， it has been used on percussive reverse circulation drilling rig with air-lift reverse circulation for subway construction in Xiamen and Shenzhen with significant construction efficiency， achieving good economic and social benefits. The successful development of the new type square chisel provides a better trenching tool for the construction of underground diaphragm walls.