In this paper, co-combustion characteristics of Mengxi coal (MC) and poplar (PP) blends and the effects of biomass ash on co-combustion process were investigated. A ‘model poplar’ (mPP) was prepared with pure cellulose, hemicellulose and lignin basing on the lignocellulosic contents of PP. Thermogravimetric analyses were performed on MC, PP, mPP, MC/PP blends with 10%, 30%, 50% (wt.%) of poplar, MC/mPP blends with 10%, 30%, 50% (wt.%) mPP samples. The devolatilization temperature, ignition temperature, peak temperature and burnout temperature were used to evaluate the co-combustion behaviour of different samples. Activation energy and pre-exponential facts for different processes were determined. It is evident that the combination between components in PP and ash of PP has marked impacts on devolatization process of coal/biomass blends. It is also clear that poplar ash has a catalytic effect on the co-combustion process which makes the ignition of blends with higher biomass blending level takes place easily at lower temperatures. The ash matter in poplar lowers the decomposing rate of cellulose in biomass and the highest decomposing rate is observed at a lower temperature. Furthermore, poplar ash also affects weight loss rates of blends during combustion process. Impacts on burnout temperature of the blends are not significantly affected by blending poplar at low level. From data of active energy and pre-exponential factor, the ash in biomass reduces the reactivity of the blends at lower temperatures between 200-400°C, but makes major co-combustion process at temperature 400-600°C easily take place at lower biomass ratio.