Paper mulberry (Broussonetia papyrifera) is an important crop that can be used for the production of paper and animal feed. The aim of this study was to study the influence of ensiling density (250 and 500 kg/m(3)) and a lactic acid bacteria inoculant on fermentation parameters, bacterial communities and metabolites of B. papyrifera leaves silage. Silages ensiled at 500 kg/m(3) showed lower (P < 0.05) pH and coliform bacteria number than 250 kg/m(3). LP increased (P < 0.05) the content of lactic acid and decreased (P < 0.05) the pH value, the contents of ammonia-N and butyric acid, and the number of coliform bacteria. Higher abundance of Enterococcus and Lactococcus were observed in B. papyrifera leaves ensiled at 500 kg/m(3) than 250 kg/m(3) and Lactobacillus dominated in all LP treated silages. LP reduced the abundance of Clostridium and Enterobacter. 834 chemicals were identified in B. papyrifera leaves silage, including 155 lipids, 145 phenolic acids, 117 flavonoids, 94 amino acids and derivatives, 80 organic acids, 69 alkaloids, 56 nucleotides and derivatives, 18 lignans and coumarins, 13 terpenoids, 11 tannins. 213 differentially accumulated metabolites were identified when compared 250 kg/m(3) to 500 kg/m(3), while only 23 were detected after LP was added. These metabolites were correlated with bacterial community, especially Sphingomonas and Lactobacillus. In conclusions, ensiling density influenced bacterial communities and metabolites of the silage. Lactic acid bacteria inoculants application is a feasible way to obtain well fermented B. papyrifera leaves silage even ensiled at a relative low ensiling density. It provides insight into the metabolites, bacterial community and their relationship in B. papyrifera leaves silage. Furthermore, it might be helpful for high quality silage production when compacting machine is lacking.