The formation energies, electronic structures, and magnetic properties of a series of SnO2 systems co-doped with transition metals (Mo, Ru, Rh, and Pd) and oxygen vacancies (VO) were investigated using plane-wave density functional theory with ultra-soft pseudopotentials. The results show that the formation energy of the Mo-VO-SnO2 system was the smallest among the doped systems, indicating that the system was the easiest to form. The net magnetic moments of the X-VO-doped SnO2 systems (X = Mo, Ru, and Rh) are not zero, indicating that the systems are all in the ferromagnetic state. Among them, the Mo-VO-doped SnO2 system had an enormous net magnetic moment, highest spin polarization rate, and best ferromagnetism, making it an excellent ferromagnetic candidate material. The ferromagnetism of the doping systems originates from the strong hybridization of the 4d orbitals of the unpaired electrons in the dopants. The VO-SnO2 does not exhibit any magnetic properties. Therefore, VO doesn’t play a role in the generation of magnetism