Internal pores and fractures inside oil shale undergo complex changes during pyrolysis in superheated steam. After a simulated in-situ pyrolysis in superheated steam, the pore characteristics of 30 oil shale samples from different zones between the injection and production wells were investigated using mercury intrusion porosimetry (MIP). The results showed that the porosity of oil shale exceeded 27.65% in each position of horizontal fractures. In different types of pores, the proportions of mesopore (0.1 mu m < d <= 1 mu m) and micropore (d <= 0.015 mu m) were the largest and the smallest, respectively. Moreover, mesopores outnumbered micropores. The pore structure of pyrolyzed oil shale after convection heating was complex. After the pyrolysis in superheated steam, the proportion of the oil shale ore body with a porosity ranging from 23 to 31% was 74.95% of total. This finding proved that oil shale was transformed from dense rock to a porous medium under the effect of superheated steam.