The invention provides a method for preparing kalium sulphuricum by employing potassium and magnesium sulfate mixed ores got from solarizing the sulfuric acid magnesium salt lake containing potassium saline water salt-field as raw material, which comprises: solarizing the saline water containing potassium from sulfuric acid magnesium salt lake, getting one-stage potassium and magnesium sulfate mixed ores and two-stage potassium and magnesium sulfate mixed ores, proportioning the two ores, kalium sulphuricum mother liquid and water according to a certain propotion, proceeding thermal conversion in a certain temperature range, thermal filtering to remove sodium, cooling for crystallization to evole out medium raw material, adding water into the medium raw material, then disintegrating and conversing directly, cooling, separating, washing, and drying under a certain temperature range, and getting the kalium sulphuricum. The inventioin is characterized by the following aspects: adding no extra potassium chloride, low producing cost, high quality of the product which can be directly used as industrial raw material; employing no floating agent, removing the grinding and float choosing process, simple process, convenient operation, high benefit, no pollution to the ecological environment, increasing the utilization coefficient of the salt lake resource and realizing the resource utilization stratege of circulating economy.