This paper studied the effects of nitrogen- and phosphorus fertilization on the total phenolics (TP) and extractable condensed tannin (ECT) contents in the branchlets of Casuarina equisetifolia seedlings, aimed to approach the nutrient effect on tannin production. Under nitrogen fertilization, the TP and ECT contents decreased significantly, which supported the hypotheses of carbon-nitrogen balance (CNB) and growth-differentiation balance (GDB), but the plant nitrogen content had less change, resulting in the decrease of TP/N and ECT/N ratios. Phosphorus fertilization had no significant effects on the TP and ECT production. With prolonged treatment time, the TP content in the branchlets of C. equisetifolia seedlings increased by 9.91% - 14.32%, but the ECT content decreased by 14.32% - 298.88%. The TP and ECT had opposite relationships with organic matters content, showing that different types of tannin had different biosynthetic pathways. However, both TP and ECT had no significant correlation with nitrogen content, and thus, the protein competition model (PCM) was not supported. Under nutrient-poor condition, the TP/N and ECT/N ratios were relatively high, which would be beneficial for the improvement of defense ability, the decrease of litter decomposition ratio and nutrient loss, and the maintenance of high productivity of C. equisetifolia plantations.