Abstract: To reveal the regression relationship between the sapling increment and the rate of P fertilizer application, an experiment of P fertilizer application was conducted in a 5-years low-yield young Pinus massoniana forest. The results showed that there was a parabolic trend equation which could efficiently represent the relationship between the increment and the application amount, the reaction curve was a typical convex curve and reflected one peak in each index of increment. Each index of increment rose with the increase of P fertilizer application amount before the peak, and turned to decrease with the increase of P fertilizer application after the peak. According to the results, 0.11 kg P fertilizer per tree should be applied to trees when the average height and DBH was the maximum respectively, the maximum corresponding height and DBH in theory were 2.6 m and 4.87 cm, and respectively increased by 36.84% and 10.93% in comparison with that of CK.