The SC200 proton therapy system commis-sioned by the Hefei CAS Ion Medical and Technical Devices Co., Ltd. (HFCIM; Hefei, China) and the Joint Institute for Nuclear Research (JINR; Dubna, Russia) has made significant progress. A main radial beam diagnostic system (MRBDS) equipped with a new type of train probe was developed to satisfy the requirements of beam diag-nosis. In this paper, the detailed design of the mechanical structure and electronics system of the MRBDS is pre-sented. The electronics system, which includes hardware and software components, was tested and calibrated. The results show that measurement errors can be significantly reduced by the designed calibration procedures. The repeatability of the mechanical structure was also verified, and the experimental results indicate that the unidirectional repeatability of the positioning is better than 0.3 mm. Finally, the MRBDS was used to measure the phase lag of the particles in the beam, and the results showed a high degree of agreement with theoretical calculations, which proved the applicability and high efficiency of the MRBDS.