The development of new spin-crossover complexes provides novel promising switching materials with significant potential at the molecular level. Ter-imine-type molecules represent one of the important dasses of ligands in creating SCO-active complexes. Herein we report a family of mononuclear Fe(II) SCO-active compounds constructed from a new type of ter-imine ligand named the thio-pybox ligand (2,6-bis(4,4-dimethyl-4,5-dihydrothiazol-2-yl)pyridine, L-1). Through the variation of counteranions, some cases display complete SCO and with T-1/2 near ambient temperature. Among them, annealed [Fe-II(L-1)(2)](ClO4)(2) [1(ClO4)] shows T-1/2 down arrow and T-1/2 up arrow as 319 and 349 K, respectively. The wide thermal hysteresis of Delta T = 30 K originated from the weak interaction between complex cations and counteranions in the crystal lattice. Impressively, its high-spin population can be increased considerably by annealing at high temperature. The metastable high-spin phase is stable in the successive magnetic measurements and would gradually relax to its initial state with high population of low-spin configuration at ambient temperature. In acetonitrile-diluted solution, 1(ClO4) still maintains SCO with an estimated T-1/2 at 240 K. Differential scanning calorimetry disdoses the structural phase at around 355 K in the first heating process and the increase in the high-spin population concomitant with annealing was also corroborated by Fe-57 Mossbauer measurements. Additionally, the influences on SCO by counteranion and ligand structure are investigated, which show that the fine tuning of complex structures can affect the behavior of the spin state significantly. Finally, magneto-structural correlation studies were performed on the structures of 1(ClO4) and its oxygen analogue at multiple temperatures. The analyses of some structural parameters, including terminal N center dot center dot center dot N donor separation, bite angle, patulous angle, and the root mean squared deviation indicate that the replacement of the oxygen atom with a sulfur atom can effectively improve the flexibility and release the steric strain and thus tune the SCO toward ambient temperature. Our research demonstrates the rational design of the ligand can lead to new SCO-active compounds with high performance.