Abstract: Gulu hot spring is located on the west bank of Sangqu river ,Gulu town, Nagqu county of Tibet .The elevation is about 4700～4750m. It is still intensely activing. Consisted mainly by geyserite, the sinter formed a medium scale hot spring type Cs-deposit. On the basis of investigating the characteristics of the field geology and study on chronology, this paper systematically studies the characteristics of geology - geochemistry and the ore forming process.The kinds of structure are megagrained massive,brecciform , cataclasis and foliated, fine-grained pore ,campact massive and oolitic. The kinds of texture are grained, colloform,organic, corrosion, rectangle, coating and tubular etc. Organic texture exists in all ore-forming stages of Gulu Cs-deposit. The main mineral of geyserite is Opal.But different ore-forming stages have different kinds of opal. In stage 2 and 3, there are CT-opal and Quartz.In stage 4 and 5,is A-opal，but there is no CT-opal.In opal, the highest content of SiO2 is 97.48%, ∑(CaO + Na2O + MgO) is 3.2% and Cs2O is 3.57%.From early to late stage, the content of SiO2 gradually falls while contents of Cs2O rise. From stage 2 to 4, the contents of Cs are lower than the standard of Chinese pay grade（500×10-6）.In different stages, sample locations in the figure of Fe-Mn-Al are different.In all 39 samples, 30 samples are located in biological sediment region,9 samples in hydrothermal sediment region.The contents of REE is lower than that of North America shale . The contents of Cr-Zr and Th-U are relatively lower than that of ordinary hydrothermal sediment. From early to late stage, δ30Si gradually falls while δ18OSMOW rises. δ13CPDB and 87Sr/86Sr of Gulu geyserite show the organic characteristics. On the basis of deep geological process, Gulu Cs-deposit is formed mainly by organic process.