The article demonstrates the method of engineering defects into molybdenum disulfide surface to facilitate dithioglycol conjugation. The molybdenum disulfide (MoS2) nanosheets containing phosphorus, nitrogen and silicon elements were successfully synthesized by a "thiol-ene" click reaction between defect-rich MoS2 nanosheets with sulfydryl groups and ene-terminated hyperbranched polyphosphate acrylate (HPA) and characterized by FTIR and XPS to confirm covalent functionalization throughC-S bond. The influence of MoS2-HPA on combustion behavior of unsaturated polyester resin (UPR) composites was investigated by cone calorimeter, demonstrating the obvious reduction of 43.2% and 39.6% in Peak Heat Release Rate and Total Heat Release. TG-IR-MS coupling technique was utilized to reveal thermal degradation and identify toxic volatiles such as CO, benzene, methylbenzene. Based on gaseous and condensed phase, we reasonably proposed the flame-retardant mechanism of MoS2-HPA/UPR composites. Moreover, mechanical properties and thermal stability of UPR matrix have significantly improved, due to the incorporation of MoS2-HPA hybrids. It is anticipated to exploit covalent functionalization of MoS2 nanosheets and MoS2-based composites with high performance. (C) 2016 Published by Elsevier B.V.