The chip-scale atomic clock technology based on coherent population trapping (CPT) atoms is reviewed, including the basic principle of CPT atomic clock, Ramsey-CPT atomic clock technology, schemes of CPT atomic clock suitable for miniaturization, and the development and current status of the chip-scale CPT atomic clock. The key technologies， such as laser frequency modulation, Ramsey technology, left and right circularly polarized light pumping (push-pull), laser and microwave frequency stabilization, and micro optical-electro-mechanical system (MOEMS), are analyzed and discussed. Finally, it is concluded that the CPT atomic clock is developing in the direction of low power consumption, chip-based and high clock precision.