【Objective】 The effects of different thinning retention densities and different ratios of N and P fertilizers on the growth and timber assortment structure of middle-aged Chinese fir (Cunninyhamia lancelata) forests were analyzed.【Method】 Nine standard plots were set up in a 9-year-old Chinese fir plantation with 23 site indices at the Yangkou National Forest Farm in Fujian Province. An orthogonal experimental design was used to carry out matching experiments of thinning and fertilization. The thinning retention density was 1 200, 1 500 or 2 250 trees/hm²; the N fertilizer application rate was 0, 100 or 200 g per tree; and the P fertilizer application rate was 0, 250 or 500 g per tree,this experiment lasted for four years.【Result】 The results show that different thinning densities and N and P application rates had no significant effect on tree height growth. Different thinning retention densities had a greater impact on the average DBH growth of Chinese fir forests, and revealed a significant difference between the minimum density and the maximum density. The application rate of N fertilizer and P fertilizer had little effect on the average DBH growth of Chinese fir forest, and there was no significant difference between different N and P application rates. There was a significant difference in the average volume increase per plant among different thinning retention densities, and the application of N and P fertilizers had little effect on the average per tree volume increase. The increase in stand volume among stands of different densities was not significant, but higher levels of N and P application rates could promote a significant increase in stock volume increase. The low thinning retention density significantly affected the large-diameter wood yield, and it was highest under the retention density of 1 200 trees/hm². Higher yields of medium diameter timber were obtained at retention densities of 1 500 and 2 250 trees/hm². A high level of N fertilizer application promoted an increased large-diameter wood output, while a high level of P fertilizer application promoted an increased medium-diameter wood output, but the result was not significant.【Conclusion】 In general, compared with the application of N and P fertilizers, the effect of density regulation on the growth and timber structure of Chinese fir middle-aged forests is more significant. The lower forest density is key for cultivating large-diameter fir timber, and the effects of N and P fertilizer application require long-term observation.