In this work, the dielectric functionally graded material (ε-FGM) was initially employed to improve the electric field distribution along spacer surface. Taking the permittivity distribution as the object, a global optimization algorithm was proposed in this paper, which usesd the maximum electric stress as a feedback to update the permittivity distribution. Using the APDL language incorporated in ANSYS combined with MATLAB, the influences of step length and layer number on the field were studied. The results show that the electric stress along spacer surface becomes smoother with the decrease of step length and the increase of iteration number. Correspondingly, the permittivity range is significantly enlarged. The high electric field area of spacer is shifted from the electrode and triple junction to the middle part, and the utilization ratio along the lower and upper surface reaches 0.75. The results provide useful references for spacer design.