CYCIAE-100 is a compact proton cyclotron with the maximum energy of 100MeV; it was designed at CIAE (China Institute of Atomic Energy) as a main part of BRIF (Beijing Radioactive Ion-beam Facility). A spiral inflector and central region were designed to reduce beam loss and inject good quality beam into CYCIAE-100 from an external H ion source. The behavior of beam with the fourth harmonic RF mode in central region was studied through numerical calculations, including accelerated centering beam, transverse and RF acceptances and so on. The radial motion study was carried out to minimize the centering error; and the acceptance of central region was calculated. The design procedure and results will be presented. Results show that this design can meet the requirements of 100MeV compact cyclotron.