The existing leakage models，which include smooth plane leakage model，rough and waveform surface leakage model，and tapered-face leakage model，can not be applied to describing the active state of contacting mechanical seal.In these models，only prime microscopic surface topography and constant leakage rate are taken into consideration.Based on fractal geometry theory，a leakage model was established with consideration of the change of microscopic surface topography and time-correlation of leakage rate.The relationship between leakage rate，the change of microscopic surface topography of seal surface and leaking channel under operation conditions was described quantitatively by fractal parameter with unattached dimension.There existed an optimum fractal parameter during the mechanical seal running，which corresponded to the lowest leakage rate.The research results helped to ascertain the roughness and waviness of seal surface，and to select machining methods of sealing members.According to the leakage model presented in this paper，the leakage rate of contacting mechanical seal can be forecasted，which may be used as reference in the maintenance and operation of mechanical seals.