A novel hybrid material derived from wood and acid nanospheres was synthesized by facile in-situ self-assembly of phosphotungstic acid (HPW) and triethylamine (TEA) at room temperature. The uniformly dispersed amphiphilic TEA-PW nanospheres furnished the wood with controlled particle sizes, and the acid capacity of the material could be altered by varying the molar ratio of HPW and TEA. Furthermore, the TEA-PW/wood hybrid had good anti-leach properties, which were attributed to the acid nanospheres' amphiphilicity. Notably, the hybrid material displayed excellent flame-retardant characteristics even with a low dose of the TEA-PW nanospheres (only 18.6 mass %), resulting in a UL-94V-0 rating and a 41.4% and 44.8% decrease in the pk-HRR and TSR of the untreated wood, respectively. Effective flame retardation was primarily attributed to synergistic effects, molecular firefighting properties, and catalytic carbon reactions in the condensed phase high-density nitrogen-phosphorus-oxygen cross-linking network carbon layer, as well as the capture or dilution of flammable gases by free radicals. In conclusion, this hybrid material may provide new possibilities for the development of fire-resistant woods.