Abstract: After the production of the gradient coil of the magnetic resonance imaging system is completed, magnetic field testing is required to confirm whether the assembly accuracy meets the magnetic field requirements. Because the passive magnetic field Bz satisfies the Laplace's equation and is a Harmonic function; According to the extreme value principle of Harmonic function, the maximum or minimum value of Bz can only appear on the boundary, so the observation points of the magnetic field are generally selected on the surface of the spherical imaging area. For superconducting magnets used for whole body magnetic resonance imaging of the human body, the shimming zone is generally selected as a spherical area with a center diameter of 40−50 cm as the shimming target area. Only the field value of the 40−50 cm spherical surface needs to be measured, and the spherical harmonic coefficient obtained after data processing is used to determine the magnetic field performance of the gradient coil. There are many testing principles and methods for magnetic fields, so there is no unified way and method in the field of commercial applications. This article is based on the Gaussian Legendre integral numerical algorithm, measures and analyzes the magnetic field performance of gradient coils by building a data acquisition system. This article applies numerical analysis methods to calculate the spherical harmonic coefficient of the magnetic field using discrete test data, providing a feasible method for the production and testing of gradient coils.