To make clear whether Mongolian pine (Pinus sylvestris var. mongolica) plantation is limited by soil phosphorus (P) supply in southeast Horqin sand land and to find out the best leaf indicator of soil P supply, the concentrations of total P, inorganic P and organic P in the needles of different age of P. sylvestris var. mongolica and the soil available P were analyzed. The results showed that in the study area, soil available P was rather low (0.12-0.63 mg x kg(-1)), and had significant correlations with the inorganic P (cPi) and total P (cPt) concentrations in the current-year needles of P. sylvestris var. mongolica. The significant correlation between soil available P and needle cPt derived from the significant correlation between cPi and cPt. Compared with cPt, cPi could reflect the level of soil P supply more accurately and directly.