Magneto-structural correlations in a series of lanthanide polyoxometalates (POMs) with pentagonal bipyramidal symmetry, namely, [Ln(2)(NMP)(12)(PW12O40)]-[PW12O40] (NMP is N-methyl pyrrolidone), were studied in detail experimentally combined with theoretical calculations. Furthermore, two types of Dy-based complexes with pentagonal bipyramidal symmetry were built to discuss the dependence of the theoretical energy barriers with the axial Dy-O bond lengths when the magnetic axes in ground Kramers doublet are along the axial orientation or on the equatorial plane. A meaningful conclusion was put forward for designing such Dy-based SIMs with high performance.