The thermal behavior of poplar-fat coal (biomass-coal) blends and organic functional structure of formed coal-char in the co-pyrolysis temperature were investigated using Thermogravimetric analyzer, Differential scanning calorimetry, Mass spectrometry and Attenuated total reflection Fourier transform infrared spectroscopy analysis. Furthermore, the interactions between poplar and coal during co-pyrolysis were deduced. The results indicate that poplar decomposed prior to the decomposition of fat coal is not surprising, but results also indicate that the presence of poplar enhanced the thermal decomposition of fat coal at low temperature. The occurring interactions showed positive and negative effects with increasing temperature, which could be explained by chemical reaction and physical interaction, respectively. In the blends, these interactions lowered the apparent activation energy and frequency factor. The added poplar had a positive effect on decomposition of the organic functional groups. Interactions would indirectly cause higher hydrocarbon-generating potential and thermal maturity, and reduced aliphatic chains length and aromaticity. The synergistic effects between fat coal and poplar during co-pyrolysis occurred mainly at lower temperature. For the better application of poplar-fat coal blends, the suitable blending ratio of poplar to coal is below 16%. In addition, the pyrolysis temperature should be kept in 345-390 degrees C to obtain biomass-coal tar. (C) 2019 Elsevier Ltd. All rights reserved.