[en] In order to satisfy the requirement of beam injection and extraction of CSR, four magnetic septum will be used in the accelerator. In the design, the shape of the lamination is optimized by 2-D magnetic field calculation. Also, in order to decrease the leakage field and obtain a satisfied field distribution, a special auxiliary coil and a shield iron plate are considered and as well as the construction of the septum leg is treated in a special way. As a result, the leakage field is almost zero and the inhomogeneity is less than 0.1% even in the area very near to the septum leg