Abstract: To explore the optimal nitrogen and phosphorus foliar concentration for the growth of Pinus yunnanensis seedlings, two-year-old seedlings of P. yunnanensis were used as experimental material, 9 treatments of nitrogen and phosphorus combined foliar spraying with different nitrogen and phosphorus concentrations, 2 factors and 3 levels were applied. The root growth indexes, including total root length, average root diameter, root surface area, root volume and root biomass, were measured in August, October and December of the same year after fertilization, and the regression analysis and correlation analysis were conducted. The findings revealed that there were some differences in the response of different root growth indexes to nitrogen and phosphorus combined foliar combined application. The total root length and average root diameter of P. yunnanensis seedlings were significantly different among different treatments during the measurement period. The root surface area in August and December, as well as the root volume in August, exhibited significant differences among various treatments, while the root biomass showed no significant differences during the measurement period. Compared with the treatment without nitrogen and phosphorus application, nitrogen and phosphorus application increased the total root length and root surface area during the whole measurement period, the mean root diameter and root biomass increased in the early stage(August) and decreased the later stage(October and December), and the root volume increased first and then decreased and then increased. During the whole measurement period, the total root length, root surface area and root volume increased first and then decreased, while the mean root diameter and root biomass increased gradually. The relationship between total root length, root diameter, root surface area, root volume, root biomass and nitrogen and phosphorus dosage of P. yunnanensis seedlings showed a bell-shaped curved pattern. There was a peak yield of each root growth index. Before the peak value, each root growth index increased with the increase of nitrogen and phosphorus dosage, and after the peak value, each growth index decreased with the increase of nitrogen and phosphorus dosage. Correlation analysis showed that the correlation between nitrogen and phosphorus and root growth indexes was different, and the overall performance was positive correlation in the earlier period(August). The effect of nitrogen and phosphorus combined application on root growth was more obvious in the early stage, and the effect of nitrogen and phosphorus combined application was obvious at medium concentration(nitrogen 0.25%, phosphorus 0.2%).