The space cold atom clock, developed by Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, was launched into space on September 15, 2016 with “Tiangong-2”. It is the world first cold atom clock which had operated in orbit and carried out scientific experiments. Since launched, the clock had been operating successfully in orbit for more than two years. During the operation in orbit, the laser cooling and the manipulation of cold atoms in microgravity were investigated, and a key technology on the interrogation of microwave with cold atoms in microgravity was demonstrated. The successful operation of the cold atom clock in orbit would help to establish a frequency standard in space, and give new opportunities in space fundamental physics.