Abstract: The present work focused on the evolution of char surface structure during pyrolysis of sewage sludge. It shows that the char pore structure is developed greatly as the release of volatile. Both high temperature and low temperature pyrolysis increase the pore with a diameter of 3.75nm. The fractal analysis of char surface with N2 adsorption shows that there are two fractal dimensions: FD-1 and FD-2, which are mainly account for the larger pore (>0.86nm) and ultramicropore (<0.86nm) fractal characteristics, respectively. The increase in pyrolysis residence time increases the FD-1 and FD-2 firstly, and then tends to be stable. The FD-2 increases greatly at high temperature pyrolysis compared with that at low temperature, which shows a great development in ultramicropore, while FD-1 is not the case.