The Qarhan potash-magnesium salt deposit, which consists from east to west of four playa districts, namely Huobuxun, Qarhan, Dabuxun and Bieletan, may be divided from bottom to top into three salt beds(S_1, S_2 and S_3), with potash salts mainly occurring in S_3. The paper lays emphasis on the evolutionary history of the deposition of Salt Bed S_3 and the modes of formation of potash salts in the Qarhan playa.The analysis of the data obtained from more than 170 boreholes and the geochemical studies on bromine, magnesium and calcium reveal the existence of four concentration stages and three dilution stages during the ormation of the Upper Salt Bed S_3. In the process of concentration, evaporation of brine and precipitation of salt minerals dominated, which obeyed the regularity indicated by the phase diagram of the quaternary system Mg-Na-K/Cl~-H_2O. In the dilution stages, however, mixing of brines, dissolution of salts(especially the playa) and mechanical clastic deposition played the leading role. The prerequisite for mixing in the Qarhan salt deposit lies in that chloride type brine of the northeastern part coexists with magnesium sulfate type brine of the southwestern part in the same salt lake. The appearance of bassanite has verified the process of mixing. The dissolution refers to the partial dissolution of the salt beds originally deposited. Particularly in the last dilution process, approximately 200 km~2 of the playa north of the Tuanjie Lake had been dissolved before a new-born lake took form.Solid potash salts were formed in three ways:(1) The surficial brine of the Dabuxun lake coexistent with the playa had an intercompensational relationship with the intercrystalline brine in the playa. As a result, K~+ ions were collected into the lake, and layered Carnallite precipitated from lake brine during the seasons of evaporation, which was accelerated by the action of wind.(2) Disseminated potash-magnesium salts were formed by further concentration of intercrystalline brine.(3) Dissolution and reformation of the playa resultedin the formation of a new-born salt lake, in the drying of which brine collected the potash-magnesium salts originally dispersed over the playa into local low-lying areas, forming finally stratoid potash-magnesium salts.