The bio-drying characteristics and its influencing factors of paper mill sludge (PMS) were investigated detailedly, by means of the heat generated by aerobic degradation of the organic substances in the PMS. In the orthogonal experiments, the good results were achieved with the followed optimization technics: starch (25.0g/500.0g), sawdust (40.0g/500.0g), inoculation (15.0mL/500.0g) and KH2PO4 (5.0g/500.0g). During bio-drying, the matrix temperature increased to 47.2°C within 12-24h rapidly under the given operation parameters, and the maximum was about 48.0°C. In the whole process the pH changed in the range of 6.11-7.87. The quantity of amylolytic bacteria reduced to the minimum in the first day, and the amylolytic bacteria grew well until the process of bio-drying finished. The ATP content was increased drastically in the first day and peaked in the fifth day, with the maximum ATP content was about 6.4×10-6μmol/g. When bio-drying of PMS was finished, the VS content and moisture content (MC) reduced from 58.4% to 49.5% and 62.2% to 50.3% respectively.