Abstract: As the key components of rapid cycling synchrotron of China Spallation Neutron Source (CSNS/RCS) collimation system, secondary collimators are used to absorb the unconstrained halo particles which are scattered by primary collimator's scrapers, they should meet the special requirements of high stability, ultra-high vacuum, high positioning accuracy and so on.Based on the design and manufacturing experience of primary collimator, the absorber which is the key part of secondary collimators is designed by using ANSYS and FLUKA, and cooling by water pipe is adopted.Combined with the control system, transient analysis has been done to ensure the feasibility of the design.