China Spallation Neutron Source (CSNS) accelerator consists of a 80 MeV linac and a 1.6 GeV Rapid Cycling Synchrotron (RCS), which is designed to produce beam power of 100 kW with a repetition rate of 25 Hz. For such a high intensity RCS, beam loss and control are of primary concern. A two-stage collimation system is designed to localize the beam losses in a restricted area, and keep the uncontrolled losses less than 1 W/m at the other part of RCS. The detailed design of the beam collimation system is presented, including the compare among different schemes. Key issues which affect the collimation efficiency are analyzed, and the collimation efficiency and beam loss distribution are studied by using the code ORBIT. INTRODUCTION The CSNS [1] requires a total number of 1.56×10 protons for the target beam power of 100 kW. In designing the RCS, one of the primary concerns is machine component radioactivation caused by uncontrolled beam losses [2]. To allow hands on maintenance, beam loss around the machine should not exceed 1 W per meter. Another important concern is the beam loss during the single turn extraction. Smaller beam emittance at extraction allows less exigent kicker strength and small beam loss at extraction. In order to achieve the low loss requirement around the ring and well constrained extraction beam extension, a two stage collimation system is designed to localize the beam losses in well shielded regions of the machine [3]. Table 1: The Main Parameters of the CSNS Ring Parameters Symbol, unit Value Inj./Ext. energy Einj/Eext, GeV 0.08/1.6 Circumference C, m 228 Beam population Np, ×10 1.56