Rainfall can alter soil moisture status and promote transpiration in forest trees, but the effect of rainfall amount and its duration on sap flow and its environmental control mechanism remains unclear. Therefore, the poplar (Populus×euramericana) plantation of a riparian ecosystem free from soil water stress was selected as the research object in Gongqing Forestry in Shunyi District of Beijing, a semi-arid and semi-humid zone in northern China. During the growing seasons of 2019 and 2021, the TDP sensors was uesd to measure sap flow, as well as the environmental conditions such as climatic factors and soil water content. Based on the results of statistical analysis of long term (2016—2017,2019 and 2021) rainfall data in the region, the event with an interval of two rainfall pulses more than 22.5 h was divided into two separate rainfall events. According to the rainfall amount and duration of the rainfall event, the events with median ±1.5 times standard error were defined as common events, while the events with a cumulative probability greater than 90% were defined as extreme events. The results showed that: (1) Solar radiation was the only environmental factor that significantly controlled sap flow in this poplar plantation during the growing season (partial correlation coefficient r_p = 0.539). Vapor pressure deficit, wind speed and soil moisture content were not correlated with sap flow (p > 0.533), and this environmental control did not change before and after the rainfall event; (2) Sap flow decreased with the increase of rainfall amount after the rainfall events (R²=0.78, p = 0.004), but there was no significant correlation with the duration of rainfall events; (3) There were no significant differences in sap flow over time on the half-hourly scale following both common events and extreme events (p ≥ 0.264), but the dominant environmental control factors after the four types of rainfall events were not identical. Specifically, solar radiation and vapor pressure deficit always significantly affected half-hourly sap flow (r_p ≥ 0.374), while soil water content only significantly increased half-hourly sap flow after common and extreme rainfall duration events (r_p ≥ 0.215). Wind speed significantly suppressed half-hourly sap flow after common rainfall amount events (r_p = -0.258), but significantly enhanced half-hourly sap flow after common and extreme rainfall duration events (r_p≥ 0.183). The results of the study provide further insight into the influence of rainfall characteristics on sap flow and its biophysical control mechanisms, as well as improving the modelling and assessment of ecohydrological processes under climate change.