In order to uncover the mechanism of phosphorus(P) efficiency in utilization in Eucalyptus,we investgated the sparingly-soluble phosphates uptake and physiological response of root system and architecture to P deficiency condition at seedling stage using ‘Guanglin 9’ by hydroponics and pot-culture experiments.It was suggested that:(1)P uptake by Eucalyptus seedlings root was up to 4.24 mg/plant under low P condition of AlPO₄,which was similar to those under low P condition of water-soluble P and Ca₃(PO₄)₂.(2)Formation of shallow root was induced by P deficiency and applying P fertilizer at the topsoil (0～20 cm),which resulted in the obvious increasing ratio of root surface and root tip number between topsoil and subsoil (20～40 cm) compared with normal P treatment.(3)15 days after low P stress,H⁺-ATPase activity improved significantly in Eucalyptus seedlings and the bromocresol purple indicator turned yellow at root tip.Secreting activity of acid phosphatase from root significantly improved under P deficiency related to normal P treatment,and further improved with the extension of low P stress (10,15,20 days).Moreover,Al toxicity and P deficiency stress can significantly induced the secretion of oxalic acid from root,and the secreting amount of oxalic acid under both Al and P deficiency conditions was significantly larger than that under the control and P deficiency conditions.These results indicated that strong uptake ability of sparingly-soluble phosphates,shallow root system induced by P deficiency and both Al toxicity and P deficiency conditions,acidification of root tip and secreting acid phosphatase and oxalic acid from root possibly contributed to the mechanism of resistance to Al toxicity and P deficiency in acid soil in Eucalyptus seedling.