To investigate the impact of kinship on the growth of Chinese fir under conditions of low phosphorus (P) availability and determine if kin recognition behavior exists Chinese fir roots, an indoor controlled pot experiment was conducted. Two half-sib families (No.32 and No.28) of Chinese fir were selected as the experimental subjects. Using the ¹³C isotope labeling technique, plants from both the kin (No.32+No.32^*, ^* labeled with ¹³C isotope) and non-kin (No.28+No.32^*) groups were established under different levels of P supply (1.0 mmol/L KH₂PO₄, P₁ and 0 mmol/L KH₂PO₄, P₀). A single planting of Chinese fir (No.32^*) served as the control (CK). In the different treatments, the aboveground parts of Chinese fir (No.32^*) seedlings were isotopically labeled with ¹³C. Growth parameters, including root length, surface area, volume, and average diameter, were measured for Chinese fir seedlings under different kinship neighboring trees. Additionally, the differential patterns in ¹³C abundance were determined for roots, stems, and leaves of neighboring seedlings of No.32^*. The ¹³C abundance in neighboring plants of different kin of No.32^* exhibited significant heterogeneity. Under P-deficient conditions, the ¹³C abundance in the non-kin group (No.28) was significantly higher than in the kin group (No.32), indicating that low P stress facilitates carbon exchange between unrelated individuals of Chinese fir. Conversely, under low P stress, the total biomass of the non-kin group (No.32^*) was 31.3% lower than that of the kin group. Varying levels of P supply significantly altered the growth pattern of Chinese fir and its neighboring plants. The total biomass of Chinese fir adjacent to non-kin neighboring plants decreased, accompanied by a reduction in root-to-shoot ratio and weakened growth and development. Under low P stress, the non-kin group (No.32^*) exhibited a 46.0% increase in root surface area compared to the CK group (No.32^*), while experiencing a 74.7% decrease in root volume. Under low P stress, non-kin neighboring plants promoted an increase in Chinese fir root surface area, enhancing P acquisition capability and strengthening its competitive ability in the underground environment. The total P content of No.32^* in the non-kin group was not significantly different from that in the CK group (No.32^*), while that in the kin group was significantly lower. The total P use efficiency (PUE) of No.32^* in the non-kin group decreased by 35.6% compared to that of the kin group (No.32^*). Therefore, under P-deficient conditions, kin neighboring plants promoted an increase in Chinese fir's PUE, alleviated its P deficiency stress, and helped maintain its normal growth and development under adverse conditions. This study provided evidence of kin recognition in Chinese fir and its neighboring plants based on ¹³C exchange. Under low P stress, Chinese fir exhibited increased mutual responsiveness and enhanced PUE when faced with kin plants while demonstrating stronger competitiveness against non-kin plants. The study demonstrates that under nutrient deficiency, Chinese fir adopts different response strategies depending on the kinship of neighboring plants, thus providing a theoretical basis for improving the management of Chinese fir plantations, whether intraspecific or interspecific.