Pitches with three different softening points (SPs) were obtained from the same original coal tar purified in the laboratory by centrifuge, hot filtration, and distillation. The effect of temperature and residence time on thermal polymerization of the three types of pitch was investigated using a laboratory autoclave. The differences in the composition of the three types of pitch was established by elemental and thermogravimetric analyses. The results show that the purified coal tar with low quinoline insolubles (QI) of 1.8g/kg (0.18wt%) was obtained by centrifuge and hot filtration treatment at 348K, with a 1000-mesh filter. The coal tar recovery is approximately 800g/kg (80wt%). The rate of thermal polymerization is influenced by the SP of the original pitch controlled by the distillation conditions. The QI of the modified pitch decreases (from 3.5 to 1.8g/kg, 0.35 to 0.18 wt%) in the initial reaction stages at low temperatures (below 683K) and increases at higher temperatures during the thermal treatment. The SP, coking value (CV), and toluene insolubles (TI) of modified pitches increase monotonically with temperature. A modified pitch with a low QI of 1.8g/kg (0.18wt%), a SP of 366K, a TI of 234.2g/kg (23.42wt%), and a CV of 485g/kg (48.50wt%) was prepared from the original pitch with an SP of 319K by thermal polymerization at 683K for 1h.