In order to improve thermal and flame-retardant as well as mechanical properties of epoxy resins efficiently, a new P/N/Si-containing nano-filler called 2,4-(bis-polyhedral-oligomeric-silsesquioxane)-6-[(bis-9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-yl)amine]-(p-4-hydroxypheny1)-1,3,5-triazine (abbrev. PDT) was synthesized by nucleophilic aromatic substitution reaction and was combined with curing-agent to prepare epoxy resin composites, incorporating both nanocaged framework of Si-O and rigid phenyl-rings from triazine and DOPO into the epoxy matrix at molecular level. The prepared EP-PDT composites were characterized by means of techniques such as TGA-IR, Raman, SEM, EDX analysis, Py-GC/MS and XPS etc. It showed that all samples had a single T-g, indicating that these epoxy composites were homogeneous phase for long-term use. Both the LOI value and char yield increased with the increasing of PDT content, while the corresponding PHRR and THR data of EP composites dropped. SEM revealed that PDT in EP composites self-assembled into nano-sized particles, resulting in an enhanced mechanical property. Py-GC/MS and XPS characterizations showed that PDT could release free radicals and promote the char formation deriving from the synergic effect of phosphorus, nitrogen and silicon during combustion, which resulted in the formation of quenching and barrier effects to endow the EP composites with excellent thermal stability and flame retardancy for potential applications in many fields.