The open-shell single covalent bond composed of two electrons is unstable under normal conditions, because the closed-shell electronic configuration is generally beneficial to minimize the energy of the system. This classical rule always governs the chemical bonding of s- and p-block homonuclear diatomic molecules, such as the stable σ2 electron-pair bonds in hydrogen. In this work, surprisingly, we found that the diversified open-shell single bonds between two f-block atoms (e.g., thorium) can be stabilized within a tight "carbon-confined-space" using relativistic quantum chemical calculations. We first identified a stable dithorium endohedral metallofullerene (EMF), Th26+@Ih-C806-, with a Th-Th distance of 3.803 Å inside the Ih-C80 cage, which displays a unique spin-polarized σ1π1 2-fold single-electron Th3+-Th3+ bond that is collaboratively dominated by 5f6d7s7p orbitals. The Th3+-Th3+ bond can further evolve into a 5f6d dominated spin-polarized π2 configuration by compressing the Th-Th distance further down to 2.843 Å, within a smaller Ih-C60 cage. On the other hand, elongating the Th-Th distance to 4.063 Å by encapsulating Th2 into a long diametric D3h-C78 fullerene returns the Th3+-Th3+ bond to the normal closed-shell (6d7s7p)σ2 form. Hence, a new rule is unambiguously revealed through the carbon-confinement induced spin-polarization of a single bond. The key point of this rule is the size of the carbon cage, because the squeezed effect is conducive to the effective overlap of the Th(5f) orbitals, reducing and further reversing the original large singlet-triplet energy gap of the Th26+ unit. This discovery provides pioneering guidance for exploring new chemical bonds and thorium-based endofullerenes.