Abstract: Determinations by thermal dissipation probes were conducted from June 3 of 2010 to June 6 on sap flow of Robinia pseudoacacia in Xishan Forest Farm of Beijing, as well as on environmental factors. The results showed that total solar radiation and soil temperature were the main factors to sap flow. The order of correlation coefficient of environment factors was soil temperature>total solar radiation>wind speed>air temperature>vapor pressure deficit>air relative humidity, among them, air relative humidity was negatively correlated with the sap flow, and the others wer positively correlated. Sap flow of R. pseudoacacia had a time-lag. The curve fitting with the environmental factors were cubic curve. The multiple regression models were also established.