Abstract: The performances of undulator generally have multiple specification requirements, which are often conflicting. It is difficult to accurately estimate the shimming at magnetic field shimming, resulting in time consuming and inefficiency. In order to solve the problem, in this paper a multi-objective genetic algorithm was applied to calculate the shimming of undulator magnetic field, and the magnetic field shimming of the undulator U38-S was finished. The calculation model was set up according to the shimming structure and the main problems in the calculation model were expounded. At last, the magnetic field shimming process of U38-S was given. After three times of magnetic field shimming, the trajectory center deviation, phase error and peak-to-peak error of U38-S are reduced to 0.15 mm, 1° and 0.49%, respectively. Besides, because of the strong expansibility of the genetic algorithm, the method used in this paper can also be applied to the magnetic field shimming of other types of undulators.