Abstract: Loess paleosol sequence occupies an important position in the studies of the global paleoclimate and paleoenvironments. The spore pollen in loess paleosol is regarded as the direct biological evidence for the ecological and environmental changes of loess plateau, which plays an important role for reconstructing the spatial and temporal charcteristic of precipitation on the loess plateau, and for understanding the process, regulation, and developmental trend of monsoon change in the future. With the development of loess study, it is necessary to find an analysis method of high effect, short process, and lower cost in order to get a great deal of high precision pollen data. The sieving analysis method, a new way of pollen analysis, is designed to fit the study needs of loess pollen. Sieving analysis method makes use of the difference of barometric pressure of vacuum to make the sample suspend and jump in water wholly. The sample is washed fully and the pollen is departed from the organic and inorganic materials. The relative condition of vacuum decreases the block of the tiny material against the very fine sieve. The gravitational force makes the water and various pellets lash at the sieve continuously. The material less than 10 μ is easy to be filtrated out into container. The pollen and a little amount of bigger fine material is isolated on the sieve. It is easy for us to float out the clean and clear pollen sample using heavy liquid. Compared with the conventional method, the advantages of sieving analysis method are as fellows: ① Less weight of sample and the drain of chemical reagents consumption, generally, about 30g sample which is only 1 sixth to 1 eighth of conventional method's need. ② Shorter process, a process of 12 samples needs about 3 days. ③ The sieving analysis makes nearly all pollen on the sieve and loses few pollen, which increases the study precision. ④ The new method nearly doesn't destroy the pollen structure; the pollen is clean and clear. We have analyzed 69 pieces of pollen samples using sieving analysis method from Yaoxian section of Holocene and found different amount pollens in all samples. Pollen sample has little impurity. Author has counted all of 14978 pollen grains which belong to 27 families and genus. All of these families and genus include the main pollen types in loess plateau. We can initially decide from the results of pollen analysis that the grass plants make up about 93.6% and the tree plants only make up 4.7% of the total pollen amount. The pollen peak of tree plants doesn't arise over 30% in whole section. We seem to think that the grass plants dominated the whole loess plateau and there were not forest in Holocene.

Key words: loess, spore pollen analysis, sieving analysis method