The extraction system is the core component of the Rapid Cycling Synchrotron (RCS) of the China Spallation Neutron Source (CSNS). It is very important for beam striking the target accurately and stable operation of the accelerator. In this paper, firstly, the extraction system and beam extraction scheme of the RCS are introduced in detail, with emphasis on some key technologies. Secondly, the extraction beam commissioning is studied in depth, including longitudinal beam commissioning, transverse beam commissioning, and extraction beam distribution optimization. The longitudinal beam commissioning mainly refers to the accurate calibration of 8 Kickers^' timing. The transverse beam commissioning mainly refers to the matching settings between the Lambertson magnet, 8 Kicker magnets and the beam transport line mode from the RCS to the target. Finally, the extraction beam loss is studied and optimized in-depth. Various sources of the extraction beam loss are explored. The leakage field of the Lambertson magnet, extraction bunch length, flat top of kicker waveform, kicker waveform variation, and so on, are studied in depth. Some new measurement methods are also discussed in detail. At the same time, the phenomenon of super-large radiation hot spot generated at the entrance of Lambertson magnet is studied in depth to find out the source of the large beam loss and put forward the final solution to reduce the extraction beam loss and radiation dose, and then make it meet the requirements of the accelerator operation.