Abstract: In order to open out the threaten field mechanism of shelter-belt on farmland to nearby crop, and offer theoretical base to tree allocation of shelter-belt on farmland and establishment countermeasures to threaten field, the stem sap flow of 8 years' NL80105 Poplar and 11 years' I69 Poplar in growing season were measured by means of ICT2000TE based on thermal pulse technique. The results show that the intensity of sap flow of I69 Poplar was 0.244L·cm^-2·d^-1, and that of NL80105 Poplar was 0.172L·cm^-2·d^-1. Neglecting the store of moisture tree crown position, it was taken a conclusion that the transpiration intensity of I69 Poplar was relatively higher than that of NL80105 Poplar; the diurnal change of sap flow showed a tendency of low-high-low; at night the sap flow could be obvious increase in I69 Poplar and NL80105 Poplar.