By analyzing the kinetics of thermal debinding, shifting of different controlling mechanism was found in the debinding process. The definition of critical thickness was specifically put forward for the first in this article. Big critical thickness means fast initial heating can be performed, which means the time for debinding can be comparatively shorter. To avoid defects such as blisters, cracks and distortion, heating of compacts with small critical thickness should be very slow. In debinding practice big critical thickness is more favorable. Theoretical calculation of controlling steps show that critical thickness depends on the flowing factor: powder size, debinding temperature and solid loading; its directly proportional to powder sizes, its inversely proportional to temperature and solid loading rate.