The objective of this paper is to study the relationship between species diversity, stand structure and forest productivity across forest strata which provides theoretical support for sustainable forest management strategies. The coniferous and broad-leaved mixed forest in Jiaohe of Jilin Province was taken as the research object, based on the field survey data of 38432 woody plants in the 30 hm² fixed monitoring plot. The structural equation model was used to study the mechanism of forest productivity based on species diversity and stand structure diversity across forest strata. The results show that: (1) overstorey stand structure diversity had the negatively direct effects on productivity, with a path coefficient of -0.138. But understorey stand structure diversity had the positively direct effects on productivity, with a path coefficient of 0.083. (2) Overstorey species diversity had the positively direct effects on productivity, with a path coefficient of 0.099. The relationship between species diversity and productivity in understorey was not significant, and overstorey species diversity significantly promoted the understorey species diversity, with a path coefficient of 0.147. This study suggests that no sole and ubiquitous relationship between species diversity, forest structure and forest productivity, but rather that the magnitude and direction of this relationship is greatly dependent on the forest strata. Therefore, the relationship between the species diversity, stand structure diversity and forest productivity across different forest strata must be fully considered in the forest management process.