The π-π stacking interaction, one of the main intermolecular forces, sometimes leads to amazing magnetic properties. Although the concept has been raised that spin density is one of the main factors that contribute to the magnetic coupling strength in intermolecular magnetic coupling systems, it has not been confirmed either experimentally or theoretically to date. Herein we present a study on the magnetostructural data of seven unpublished Cu(II) complexes and ten reported radicals. It is confirmed for the first time that the spin density on short contact atoms is a major factor that contributes to the π-π stacking magnetic coupling strength. Based on the reported data to date, when the short contact distance is larger than the default contact radius, medium or relatively strong magnetic coupling strength could be obtained only if the spin density on the short contact atoms is greater than 0.1350; when the C···C short contact is less than the default contact radius of 3.4 Å, but not less than 3.351 Å, and the spin density is less than 0.1, neither medium nor strong magnetic coupling strength could be observed. Further, when the short contact distance decreases with a temperature drop, the spin densities on the relevant short contact atoms increase. In the complexes reported the small spin densities on the relevant short contact atoms are the major factors that result in the weak π-π magnetic coupling strength.