By first-principles calculations, the magnetism of hole doped tin dichalcogenides SnX2 (X = S, Se) monolayers is systematically studied. It is found that a phase transition from nonmagnetic to ferromagnetic ground state appears once above the critical hole density (~1014 cm-2). The spin magnetic moment can maintain a magnitude of 1.0 μB/hole with excellent stability of ferromagnetic state. Furthermore, we demonstrate that strain is very useful to modulate the DOS near the valence band, resulting in the reduction of the critical hole density to ~1013 cm-2 when the strain reaches 4% (6%) in SnS2 (SnSe2), which can be realized in common field effect transistors. Moreover, the phonon dispersion calculations for the strained SnX2 monolayers indicate that they can keep the dynamical stability under the hole doping. Therefore, the strain tunable magnetic transition in hole doped tin dichalcogenides indicates their potential promising applications in spintronic devices.