Thermal spin transport properties of the Li atom doped C28 monomolecular device were simulated by using first-principle study combined with density functional theory. By applying a temperature field, the upper and lower spins provide transport channels for the electrons and holes in the Li doped C28 monomolecular device, respectively. The thermal spin current increases with temperature increasing in the MJ1 and MJ3, but the thermal spin current increases then decrease with temperature increasing in the MJ2. The spin-dependent Seebeck effect appears in the Li doped C28 monomolecular device, and the negative differential resistance effect appears in the MJ2. The physical mechanism was explained using the Fermi-Dirac distribution and spin transmission spectrum. These interesting effects suggest that the Li atom doped C28 monomolecular device can be used as new spin nanodevices.