Alkali metal compounds exhibit novel characteristics under pressure, such as antimetallization in CLi₄, high superconductivity at 80 K in highly compressed Li₃S, and the existence of unexpected stable stoichiometries of sodium chlorides, etc., which have greatly prompted us to explore K_xS compounds at pressure. We found several stable structures with a variety of stoichiometries and proposed a phase diagram on the K-rich side first. Chemical rules established at ambient pressure are frequently violated when high pressure is applied, Na₃Cl and NaCl₃ as unusual stoichiometries of sodium chloride have been reported in high-pressure conditions. However, KS, with its counterintuitive chemical formula, has been discovered theoretically even at ambient pressure, and possesses the same stability as K₂S. The mechanism of superconductivity in Pmm K₃S is deeply investigated, comparing with the reported Pmm Li₃S. The weak electron–phonon coupling mainly contributes to the weak superconductivity in K₃S, which is fully in contrast to the mechanism of interstitial charge localization which dominates the low T_c in the reported Li₃S.