The Mengyejing potash deposit in Lanping-Simao Basin of Yunnan (MPDY) is an unique ancient solid potash deposit in China. For a long time, the deposit has been thought to be of continental sedimentary genesis with potassium derived from sea water. However, there are some researchers who hold that there existed other sources, but they have failed to present any evidence. In order to find out the source and origin of the deposit, the authors studied the component characteristics of the salt layer in Mengyejing Formation and its underlying Bashahe formation by XRD, ICP-MS, EPMA and microscopy in this paper, and pointed out that there existed two potassium sources for the deposit. The first was sea water because the average value of the trace element bromine (Br) in evaporates is 578x 10-6 which is higher than 200~ 106, indicating Br came from sea water. In addition, a small amount of marine mineral glauconite exists in quartz sandstone of lower Bashahe Formation. The second was deep hydrothermal solution, as evidenced by the existence of lots of pyrite rich in Co and Ni in grayish green mudstone of Mengyejing Formation. Furthermore, comblainite is one of the main mineral phases of fracture filling in the salt layer, and azurite and malachite that contain copper minerals were found in quartz sandstone of Bashahe Formation. The elements of Cu, Co and Ni in minerals were provided by deep hydrothermal solution because thedeposit was located in strongly active tectonic belts. Therefore, deep hydrothermal solution also provided important potassium sources for the Mengyejing potash deposit in addition to sea water.