Recently, nitrogen doping (N-doping) technology has been proved to increase Q value of superconducting cavity obviously, which lowers the BCS resistance of Nb. After N-doping, Q of 9-cell 1.3 GHz cavity can be increased to 3E10 at Eacc = 16 MV/m, while 1.5*1010 without N-doping. Since 2013, there have been over 60 cavities nitrogen doped at FNAL, JLAB and Cornell. The Circular Electron Collider (CEPC) has been proposed by IHEP in China, while requests Q=4E10@Eacc=22 MV/m for 650 MHz 2-cell cavity. Bulk Nb N-doped has been adopted for these cavities, as well as Shanghai Coherent Light Facility (SCLF). So research of N-doping has been going on since 2015. Experiments of niobium samples have showed that nitrogen concentration at niobium surface increased a lot after N-doping. After then, two 650 MHz single-cell cavities completed vertical tests, which have shown Q increase at low fields.