To understand how temperate coexisting tree species respond to water deficit, the seedlings of two coexisting species, Phellodendron amurense Rupr. And Fraxinus mandshurica Rupr., were transplanted into plastic flowerpots. Three water deficit treatments were used, W1 (60-90% field moisture capacity), W2 (40-60% field moisture capacity), and naturally occurring precipitation (CK), with the aim of investigating the effects of water deficit on the growth of seedlings and competition between them. The result showed that the relative height and diameter of the P. amurense seedlings decreased in the control groups (CP, single P. amurense seedling; CF, single F. mandshurica seedling) but increased in the mixed groups (one P. amurense seedling and one F. mandshurica seedling together) with the water deficit. The water deficit affected the plants by altering the growth of the seedlings and competition between the two species. It was shown that there were greater negative effects on P. amurense due to the manufactured water deficit than on F. mandshurica.