In the field of precision measurement and aerospace, the magnetic-grating-like displacement sensor has high requirements on the uniformity of magnetic field generated by permanent magnet. While the magnetic field uniformity of common rectangular permanent magnet is poor, which affects the high precision and reliable measurement of hydraulic cylinder displacement. Therefore, this paper designed a curved permanent magnet with a unique structure. First based on the molecular current hypothesis and the Biot-Savart Law, to derive the expression of the magnetic induction intensity of a curved permanent magnet at any point in space; then, the structure size of the permanent magnet was optimized using the Maxwell electromagnetic field analysis software, to determine the basic size of the curved permanent magnet; finally, the experimental platform is built to measure the magnetic induction intensity at the corresponding position of the permanent magnet. By comparing the measured data and Maxwell simulation data, the results show that the simulation is basically consistent with the experimental results. The curved permanent magnet can produce a uniform magnetic field, and the relative error of the uniform field does not exceed 3%. In the practical application of the Magnetic-grating-like displacement sensor, the sensitive element tests the uniform magnetic field, ensuring the signal quality for the subsequent subdivision of the sensor, thus improving the test accuracy of the sensor, which further proves the practical significance of the arc permanent magnet.