Abstract: From September 26 to November 5, 2011, the sap flow of Yulania wufengensis trees including cold-resistance type (HK) and non cold-resistance type (HF), Y. ‘Sunspire’ (HY), and Yulania × soulangeana (EQ) which were introduced into Beijing four years before was monitored by Flow32 stem heat balance sensor, and, in combining with the environmental factors monitored synchronically, the changes of the sap flow before dormancy and the environmental factors were analyzed, with the responses of the sap flow to the environmental factors investigated at the scales of 0.5 h and 1 day. The sap flow of the Yulanias trees before dormancy displayed an obvious trend of declining day by day. The environmental factors affecting the sap flow could be divided into two categories, i.e., meteorological index (MI) and soil index (SI). The sap flow of the Yulanias trees had a synchronous variation rhythm with MI, and declined in parallel to SI. The combined effect of MI and SI on the diurnal changes of the sap flow was 69%-73%. At both 0.5 h and 1 day scales, the sap flow showed significantly correlations with total radiation (R_s), air vapor pressure deficit (D), air relative humidity (RH), air temperature (T_a), and wind speed (w). The sap flow showed no significant correlations with soil temperature (T_s) and soil water content (SWC) at 0.5 h scale, but had significant correlations with T_s, SWC, and day length (Z) at 1 day scale (the correlation efficient was about 0.8). Only R_s, Z, and D were included into the model at 1 day scale, but almost all environmental factors (except SWC and T_s) were included in the model at 0.5 h scale. Except for HF type, the regression coefficients of the model for the Yulanias trees at 1 day scale (0.92-0.96) were larger than those at 0.5 h scale (0.77-0.87), and the correlations between the dynamic changes of sap flow and the environmental factor were consistent, which was in accord with the fact that the HF could not overwinter in Beijing but the others could.