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The SC200 cyclotron is the critical component of the proton therapy system, which is being jointly developed by the Institute of Plasma Physics Chinese Academy of Sciences (ASIPP), Hefei, China, and the Joint Institute for Nuclear Research (JINR), Dubna, Russia. It is an isochronous cyclotron to accelerate H+ ions and extract proton beam energies of 75–200 MeV with the maximum beam intensity of 1 ~ μ A. The maximum magnetic field intensity of the SC200 cyclotron is 4.6 T, and the accuracy of measurements for the average isochronous field is required to be better than 10−4. The magnetic gap decreases from 37 mm at cyclotron center to 9 mm at the pole edge. Hence, the difficulty of designing a magnetic field measurement system is increased because of the required magnetic field accuracy and the narrow gap between the poles. A measurement system has been developed to measure the magnetic field distribution in the median plane of the cyclotron by the SENIS Hall probe. After the Hall probe is calibrated by NMR, the measurement accuracy can reach ± 4 × 10−4 under the magnetic field intensity of 5 T. The position of the Hall probe will be monitored by grating and optical encoder, respectively, so as to ensure its correct positions. To verify the performance of the mapping system, it has been tested in the SC200 cyclotron with the designed vacuum degree. The test results showed that the mapping system can run smoothly and meet the measurement requirements of the SC200 cyclotron. [ABSTRACT FROM AUTHOR]