The Nihewan Basin is located in the transitional zone of the North China Plain and the Mongolian Plateau. It is about 80km from east to west and 15~20km from north to south. The Sanggan River flows from west to east. The Nihewan Basin is one of the largest and most concentrated and most abundant areas in Paleolithic of China. Youfang site (40°13'52N,114°41'02E; 921m a.s.l.) is located in Nihewan Basin, and is one of the few places across the middle and late Paleolithic, containing both North China small stone relics and microlithic relics. The site profile (depth 1270cm) is mainly composed of wind deposition, and the deposition is continuous. The whole section (1290~20cm) is sampled continuously at intervals of 5cm. A amount of small stones were unearthed in the stratum of 1290~1150cm (56~52ka B.P.) and many fine stone products were unearthed in the layer of 870~ 20cm (45.3~27.2ka B.P.). Dating in this section is about 56~27ka B.P. (1290~20cm) dated by the OSL acrossing the MIS 3. The Youfang site can be used for the study Nihewan Paleolithic culture evolution,and provide an important basis for climate change. Based on grain size,ignition loss,combined with pollen and charcoal in the key stratum,the environmental background of transitional period between the mid and late Paleolithic is reconstructed in this paper. The result shows that (1) the average grain size (42mum) is coarse,the clay (12%) and organic matter (3%) content is lower, which is influenced by water flow,and all above shows that the typical grassland environment was the main type during the late period of mid-Paleolithic (56.0~48. lka B.P.); (2) the average grain size (27.8mum) and coarse particle content (>63mum) are the lowest (13.3%) and clay as well as organic matter content (3.6%) are highest in the whole profile in the transitional culture layer (48.1~35.2ka B.P.),showing good climate and grassland environment and tree pollen concentration is high in the early stage of this transitional culture period showing the forest was existed on the surrounding mountains, increasing effective humidity and the cold and wet climate; (3) average grain size (36mum),low organic matter content (3.3%),and grassland environment indicates the lower vegetation coverage and the dry climate during the late period of Paleolithic culture (35.2~ 27.4ka B.P.); (4) average grain size (50.4mum) and content of fine sand content (34.9%) are the highest, while the clay (7.3%) and organic matter content (2.9%) are the minimum value in the whole profile,showing the worst ecological environment and cold climate corresponding to cold MIS 2 stage during the period of 27.4~ 27.2ka B.P. During the latter stage of the transitional period between mid and late Paleolithic, climate deteriorated which might be related with H4 event,leading to ecological environment degradation and resource shortage,the ancients were forced to change their tools from small stone to the fine stones, to improve the products.