Leaching risk of heavy metal from sludge can be reduced due to solidification. But heavy metal in solidified sludge could be leached again because of Thiobacilli activity in the sludge. In order to clarify the above problem, bioreactor test was employed to enhance activity of Thiobacilli in the solidified sludge. Result shows that there was existing activity of Thiobacilli which may reduce pH and increase ORP and increase the leaching quantity of heavy metal. But further result indicated that with increasing solidification level, activity of Thiobacilli was restrained. When the solidification level reaches 5:2:2, the activity of Thiobacilli would be stop. Lengthening curing time also restrains the activity of Thiobacilli and leaching of heavy metal tends to be stabilized. When the quantity of Thiobacilli less than limited value, the bioleaching would not happen and Thiobacilli have no effect on leaching of heavy metal.