By considering the shortcomings of the traditional Halbach magnet structure which is closed and cannot be applied to the open NMR, an open and semi-circle Halbach magnet structure is proposed. Adjusting the semi-major axis of the elliptic curve which the center of magnets are placed on and calculating the flatness of the magnetic field on different heights, a magnet structure with best flatness is achieved by data fitting method. The optimizing result shows that the best magnet structure can generate a magnetic fields which flatly distributes in the horizontal direction of the 50 mm×50 mm area in the YOZ plane. The gradient is distributed in the vertical direction with gradient of 2 mT/mm. Magnetic field is uniformly distributed in the area of 4 mm×4 mm with uniformity of 1.3×10 -3 and magnetic field of 0.073 9 T. An actual magnet model is constructed based on the result of the optimal design; the measurement result is consistent with the design. The advantage of this magnet structure is that the gradient magnetic field is generated by the permanent magnet naturally, and the extra gradient coil isn’t needed, so the designing of the gradient coding system is simplified.