Soil water is an integral part of soil and an important part of “four-water” transformation. Water storage affects other environmental factors in the soil, so studying its dynamic characteristics is of paramount importance. The regression isogram can reflect the spatio-temporal distribution of water more directly. Through sampling and monitoring in the field, soil moistures of 6 representative farmland sections with depth from 5 cm to 150 cm had been obtained in the downstream area of Dagu River from April 21 to December 2 in 2017 and their dynamic changes were analyzed by regression isogram. The results showed that except for Maozibu point in the middle part of study area, the regression equations obtained in the other five monitoring points could reflect well the actual soil moistures. On the temporal scale, soil moistures of points in southern and central parts and Shaliang point in the northern part of study area decreased first, then increased, and decreased finally with time, while soil moisture in Nancun point in the northernmost part increased first then decreased with time. On the spatial scale, there were differences between the two monitoring points in the north part, as the depth increasing, soil moisture in Nancun point also increased gradually, but that of Shaliang point was consistent with the monitoring points in the southern part, increased first then decreased. Soil moisture in Liuli point in the middle part decreased first then increased. In addition, according to the section distribution of soil moisture, the position of the zero flux plane could be judged and the spatial movement direction of water could be inferred. The change of soil moisture showed a certain lag effect with the increase of precipitation. Because the rainfall mainly concentrated in July and August, except Nancun point in the northern part of the study area, soil moisture in each deep layer in other points reached a maximum in September. The above results can provide references for the proposing the reasonable regional joint regulation program of soil water and groundwater.