The C-shaped permanent magnet is used in medical nuclear magnetic resonance imager and similar analytic instrument. The magnet has three opened sides and the closed magnetic circuit consists of main magnet block, plate, side magnet block and angle magnet block on C-shaped yoke and partial connecting section of yoke. On the plate surface, there is coating of high-magnetic inductivity and high-resistivity material. The magnet has small leakage area and homogeneous inner magnetic field for high-quality imaging. The said opened magnet makes it possible for the patient into gap to change posture and for medical worker to observe and cure easily.