An incubation experiment was carried out with farmland soils severely contaminated by PCBs in the Yangtze River Delta Region under different carbon sources, ratios of carbon to nitrogen, and soil moisture contents. Starch, glucose and sodium succinate were beneficial to the growth of bacteria and fungi, thus accelerated the degradation of PCBs in polluted soils. The degradation effects of indigenous microorganism on soil PCBs were not only correlated with carbon sources, but also closely correlated with their starch, glucose and sodium succinate contents. When 1.0 gC/kg of starch, 0.2 gC/kg of glucose and sodium succinate were added to PCBs polluted soils, their effect on soil PCBs degradation increased. Effect of treatment at 10:1 carbon-to-nitrogen ratio was better than treatment at 40:1. The results also indicated that higher soil moisture content limited soil microbial growth and PCBs degradation, but soil stirring aided the growth of soil bacteria and fungi, enhanced the metabolic activities of indigenous microorganisms, and accelerated the natural degradation of soil PCBs. These results provided scientific evidence for further studies on the conditions under which natural degradation of soil PCBs occurs and could help to develop bioremediation techniques for POPs polluted soils.