Abstract: The soil acidity is the foundation that we study other physical and chemical properties of soil. At present, because of environmental pollution and acid rain, the soil is going to be acidified in most karst areas. But by sampling analysis, it is found that karst swamp soil acidity in Sichuan Jiuzaigou is between 7.72 and 9.10, and this is high to other kind of karst soil. The biggest discrepancy reaches to 4.85 units. Among 26 sampling spots, there are 17 spots with pH value exceeds 8.00, about 65% of the total. There may be four reasons for this ： geological and deep layer circle ; conversion process of soil organic materials ; the action of soil colloids and deposition of a great quantity of travertines. It is also put forward in this paper that all factors should be further studied by means of modern method such as steady carbon and oxygen isotope, local chemical exploration to hydro-deposition. All the above efforts are guarantees to sustainable exploitation and utilization as well as protection to Jiuzhaigou.