Effects of acid washing on coal characteristics are comprehensively studied using a series of experimental approaches, including SEM, XRD, BET, FTIR, TGA and in-situ measurements in a lab-scale pyrolysis furnace. Our study showed that acid-washing has marginal effects on coal crystal structure, the function groups, the temperature at the maximum weight loss rate of pyrolysis and the pyrolysis reactivity, but has significant impacts on coal micro-structure, pyrolysis activation energy, and product-gas compositions. By analyzing the SEM, XRD, BET and FTIR experimental results, it was found that the coal micro-structure becomes scattered and unstable after the acid-washing treatment, and this fundamental structural-variation leads to the reduction of overall reaction activation energy. The pyrolysis experiment concluded that the acid-washing promotes the CH4 production and inhibit CO production, while introducing negligible influence on H-2 and CO2 productions.