Abstract: Sap flow is a basic indicator to quantify plant transpiration water consumption. In this study, a continuous sap flow observation of Pinus sylvestris var. mongolica in a semiarid area of northwestern Liaoning was carried out by using the FLGS-TDP thermal dissipation sap flow probe from May to October in 2015, and the microclimate in the forest was recorded simultaneously. On the basis, the dynamic characteristics of the sap flow of Pinus sylvestris var. mongolica and its influencing factors were studied. Results showed that during the growing season, the sap flow rate of Pinus sylvestris var. mongolica varied with weather conditions, both the sap flow rate and its daily cumulative volume were decreased in the sequence of sunny > cloudy > rainy. The trend of sap flow rate followed reversed "U", and there was a phenomenon of "midday depression of photosynthesis" in sunny day. From May to October, both trunk sap flow rate and monthly accumulation amounts decreased as follows: July > June > August > May > September > October. With respect to the season dynamics, the starting hour of the trunk flow in summer was about an hour earlier than that in autumn; moreover, the trunk sap flow rate was higher in summer than in autumn. In addition, the sap flow was affected significantly by meteorological factors, in which the active radiation light, air temperature, saturated vapor pressure deficit and wind speed all affected sap flow positively, while the air humidity influenced sap flow negatively. The effects of soil temperature and humidity on the sap flow changed with time (month), weather conditions (cloudy, sunny and rainy) and soil depths (5, 10, 20, 40 and 100 cm). For example, there were significantly positive correlations between the temperature of surface soil (0-10 cm) and the sap flow (unit based on day or month) during the growing season, while there were significantly negative correlations between the temperature of deep soil (20-100 cm) and the sap flow. In addition, the sap flow increased significantly with the increase of soil moisture in May and October, while decreased remarkably with the increase of soil moisture during the growing season from June to September.