///Several tufaceous rock samples, collected from different altitude at Yong’an stone forest, were measured by Uranium-series Disequilibrium method. The results show that Yong’an Stone Forest’s formation age is approximately 250 thousand years since Pleistocene epoch, later than Lunan stone forest. Considering the history of Yong’an area geologic structure and the evolution process of Mingjiang River Basin, the results also show the developing process of Yong’an stone forest. In the process of gradual weathering and erosion of the rock overlying on Carboniferous and Permian, Yong’an stone forest is formed accompanied by interval lifting movement of crust, in the background of Minjiang River drainage area formation and Dahu Basin under erosion since Quaternary.