The research on triple helix DNA developed very rapidly in the last ten years and has become an important field of both molecular biology and genetic engineering. The current progress in this field was reviewed according to its stability and biological function. the stability of triple helix was discussed in terms of its endogenous and exogenous conditions such as chain length, base sequence, base modification, backbone structure of ODN and temperature, pH, salt, ligand binding. At the same time, its biological functions were summarized. These functions include regulating genetic expression and transcription, protecting the DNA sequence from enzymes cleavage, serving as a molecular cleaver.