Using fluid inclusions and stable isotope trace element and REE geochemical means, the writers studied the geochemical characteristics of ore fluids, source of gold and formation and evolution mechanism of ore fluids of the Mojiang gold deposit. It is concluded that the deposit is a mesothermal-epithermal gold deposit,- the ore fluids belong to intermediate to weakly alkaline sodic solutions; the water in the ore fluids was the mixture of meteoric water and magmatic water; the mineralizer, such as H2S and CO2, were mainly derived from the mantle and that gold was derived from Silurian low-grade metamorphic rocks and Hercynian ultrabasic rocks. The mechanism for the formation of the Mojiang gold deposit in the early Himalayan period is as follows- the mineralizer-rich fluids derived from depth ascended in the early Himalayan and was added to the mineralizer-poor fluids cycling in a shallow fracture system, thus the mineralizer-poor fluids converted to mineralizing fluids, which extracted a lot of gold from surrounding rocks; then the gold in the fluids precipitated at suitable places to form the gold deposits.