The design of CSNS MEBT has two objectives: (1) to match the beam both in the transversal direction and the longitudinal direction from RFQ into DTL; (2) to further chop the beam into the required time structure asked by RCS. It is very difficult and critical to control well the emittance growth and in the meantime to match and chop the beam. Firstly, the optical design is done and optimized, and the multi-particle simulations show that the maximum emittance growth is successfully controlled within 14%. Secondly, based on the different beam envelopes obtained by TRACE-3D and PARMELA, the least deflecting angle of the chopper is determined by TRACE-3D. At last, the field of steering magnet is determined through simulations.