Abstract: From observing in field,the author has found the third paleosol in Xi'an region that developed in middle pleistocene,consists of 3 paleosol layers and 2 loess beds,which is different from S_{3 of Lanzhou profile and of Baicaoyuan profile.In Xi'an region,Shuangzhucun section lies to the southern of Xi'an city and Duanjiapo section lies to the eastern of Xi'an city.Firstly,stratigraphic structure was studied,from top to bottom the structure units are S3-1,Lx-1,S3-2,Lx-2,S3-3,Lx-3.Secondly,the samples gathered from Shuangzhucun section(or Duanjiapo section) was analyzed in laboratory;magnetic susceptibility,TOC and granulometric composition were studied,and the results are as following:(1) The maximum magnetic susceptibility value(296 SI) of S3 in Shuangzhucun section and the average value are all more than those of S0,S1 in other sections,for example,Qishan profile and Fufeng profile.The results show that the soil developing of the third paleosol in this area is stronger than that in others.On the curve of magnetic susceptibility of S3,there are 3 peak values corresponding to paleosols and 2 valley values corresponding to loess beds.In the meantime,the fact that the magnetic susceptibility value of the loess(141 SI) is far more than Malan loess shows that the loess layer between paleosols isn't pure loess parent material in Duanjiapo section;the magnetic susceptibility values of the top paleosol and the bottom paleosol appear two peaks,and the middle paleosol is not same as these.(2) From the result of TOC analyzing to Shuangzhucun section,it is found that the contents of TOC of paleosol and loess are all lower(0.8%),but it still appears clearly peak and valley alternating:the peak of wave corresponds to paleosol(0.5%) and the valley corresponds to loess(0.5%).(3) Analyzing to granulometric composition of S3 from Shuangzhucun loess section shows that the composition of paleosol is distinctively differentiated from that of loess layers.Every group of grain size,Md and So appear distinctively changed in loess layer,and these changes are corresponding to quickly accumulation of loess at that time.According to the characters of stratigraphic structure,magnetic susceptibility,TOC and granulometric composition described above,as well as ancient temperature and rainfall that is recovered based on the curve of magnetic susceptibility,the period of time(330~272 ka B.P.) when the third paleosol formed may be divided into 6 stages of warm humid and cold arid climatic changes.The 6 stages show 3 climatic cycles:cycle I,include Lx-3,S3-3;cycle Ⅱ,include Lx-2,S3-2;cycle Ⅲ,Lx-1,S3-1.The climatic character was cold and arid when Lx-3,Lx-2,Lx-1were accumulating;when S3-3,the climate was subtropic warm humid;S3-2,the semi humid climate of warm temperate zone;and S3-1,the humid climate of warm temperate zone.}

Key words: the third layer paleosol, particle composition, TOC, climatic cycles, Xi'an region