Abstract: Two Bursaphelenchus xylophilus isolates (virulent and avirulent) and one B.mucronatus isolate were inoculated separately on biennial seedlings of Pinus thunbergii. Changes in water stress and activity of roots system at different periods after inoculation were measured. The results showed that at the beginning period，the seedlings were inoculated with isolates (virulent and avirulent) and the obvious symptoms had not appeared yet, the water stress of P.thunbergii experienced a process of lowering and being back up again. But when B.xylophilus inside the seedlings began propagating in large quantities and the outer was in a wilting state, the water stress of P.thunbergii lowered at a high speed irreversibly and P.thunbergii died very soon. Compared with avirulent B.xylophilus isolate, virulent B.xylophilus isolate caused P.thunbergii to lose water much faster. After B.mucronatus isolate was inoculated, the water stress of seedlings of P.thunbergii lowered and then rose again and approached to the normal level. Along with the death of B.xylophilus, the water stress of seedlings returned to the normal state. With the invasion of B.xylophilus, the activity of roots system of P.thunbergii had a rising process. At the beginning period inoculated with B.xylophilus isolates (virulent and avirulent), though the activity of P.thunbergii had decreased somewhat, still maintained a good activity of roots system and the activity would not lose until the trunk dried out, which was different from the phenomenon that the activity of roots system was gradually lost and the plant showed the symptom of wilting caused by drought. Inoculated with B.mucronatus isolate and with the death of B.xylophilus, the activity of P.thunbergii returned to the normal state after a rising trend.