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In this article, a modified design method is proposed for a kind of highly uniform magnetic field (UMF) coil, whose target region is off-center defined. Combining the traditional target field method (TFM) with the slime mold algorithm (SMA), a new intelligent algorithm with excellent searching ability, optimal parameters can be obtained to determine the structure of the designed coil. To further improve the uniformity performance in the target region, a distance factor is introduced to optimize the distribution of target points. According to the simulation results from the finite element analysis (FEA), the absolute maximum uniformity error can be optimized from 0.098% to 0.060% if the SMA is utilized, and after the optimization of target points, the volume proportion of absolute uniformity error less than $10^{-4}$ can be increased to 64.60%. An upscaled coil of 10 times is fabricated via the flexible printed circuit technology, and test results of the magnetic field generated by this coil show a good agreement with the simulation ones. It indicates that the actual uniformity error in the range of ±0.35 R reaches to 0.11%, which is acceptable when taking manufacture and measurement errors into consideration.