The coherent optical time domain reflectometry (COTDR) based on Rayleigh scattering is an important technique for the monitoring of ultra-long fiber link, for its high dynamic range and sensitivity. The principle and development of COTDR is introduced. The design method and key techniques such as the coherent fading noise suppression, measurement speed enhancement and multi-functions fusion system structure are discussed. And several important applications of COTDR are given, including ultra-long communication optical fiber link monitoring and distributed temperature/strain sensing.