Abstract: In order to study the flame retardant mechanism and pyrolysis characteristics of poplar treated by compound N-P flame retardant, we applied thermal analysis method to analyze the combustion performance of poplar treated by distilled water, Al-Si, N-P and N-P-Al-Si, numbered as A, B, C and D, respectively. Ozawa-Flynn-Wall and Coats-Redfern (modified) were used to calculate activation energy of flame retardant poplar. The results showed that A had only one pyrolysis process, with the activation energy value of this phase 65-- 70 kJ/mol. The pyrolysis process of flame retardant treated poplar could be divided into two stages. The main pyrolysis stage of D was between that of B and C. D had the lowest weight loss rate and least weight loss, and heat release rate of this treatment was also slow. There was a similar trend of weight loss of TG curves of D at different heating rates, and weight loss curves shifted to higher temperature section with the increasing heating rate. The activation energies of the first and second phase of D (120 and 240 kJ/mol) were higher than that of C (115 kJ/mol), indicating that the flame retardant efficiency was improved when Al-Si and N-P were blended.