In this paper, soil water potential was controlled in the ranges of 0-(-)20 kPa (W1), -20-(-)40 kPa (W2), -40-(-)60 kPa (W3), -60-(-)80 kPa (W4) and -80-(-)160 kPa (W5) by a suite of specially designed root-sphere osmotic irrigation technique, and the dynamics of transpiration, water suction, tissue water potential and hydraulic resistance of Fraxinus mandshurica seedlings grown under these soil water conditions were studied. The results indicated that under sub-saturated soil water condition (W1), fine root water potential fluctuated at high levels from -0.08 MPa to -0.19 MPa, which brought about the lowest soil-root hydraulic resistance (35.52 MPa x s(-1) x m(-2) x mol(-1) in diurnal average), the highest water suction rate (4.44 mmol x m(-2) x s(-1) in average from 10:00 to 14:00), and the strongest transpiration rate (5.11 mmol x m(-2) x s(-1) in average from 10:00 to 14:00). Under field capacity (W2), soil-root hydraulic resistance increased several times above that under W1, water suction rate and transpiration rate declined significantly, while the diurnal single-peak pattern of transpiration was not altered. Under orderly increased soil water stress (W3-W5), soil-root hydraulic resistance (the average from 10:00 to 14:00) increased approximately 10-24 times above that under W1, which caused the consequent low water suction rate and low fluctuation levels of leaf water potential (-1.25 MPa-(-)1.94 MPa for W5), and transpiration was seriously restricted (the transpiration rate being only 0.91-1.55 mmol x m(-2) x s(-1) at midday for W3-W5). It is concluded that soil water was not equally available to Fraxinus mandshurica seedlings in the test ranges of soil water potential, and significant water stress could occur when the soil water potential was lower than -40 kPa.