Abstract: A homogeneous magnetic field superconducting magnet with a room-temperature bore of 30 mm and a central field of 551 T for relativistic backward-wave oscillator was designed, manufactured and tested. As a result of magnetic field homogeneity considerations, the magnet is composed of three coaxial coils. All coils are connected in series and charged with a single power supply. The magnetic field homogeneity is better than ±05% from -30 mm to 30 mm in axial direction. The magnet can be operated in persistent mode with a superconducting switch. In addition, a pair of HTS current leads and a two-stage GM cryocooler with cooling capacity of 15 W at 42 K were adopted to realize a zero liquid helium boil-off. Here the design, manufacture, mechanical behavior analysis, and the test results of the magnet were presented.