According to density functional theory of first-principles calculations theory, we systematically study the electronic structures, magnetisms and stabilities of transition metal (TM = V, Cr, Mn) doped MoS2 single-layers. The results show that V-and Mn-monodoped systems each have magnetism and the magnetic moment mainly concentrates on the transition metal dopant atom, but Cr-doped MoS2 does not display magnetism. Further study on the magnetic coupling of double atoms doped MoS2 shows that the stable ferromagnetic state at room temperature is observed in the Mn-doped MoS2. However, the system shows a non-spin polarization state due to doping with V. The calculated formation energy indicates that the Mn-doped MoS2 is the most stable system. Therefore, Mn-doped single-layer MoS2 maybe have potential applications in the spin electronic devices due to its good ferromagnetism and reliable stability.