The invention relates to a prediction technology of a leakage rate of an existing contact mechanical seal. By utilizing a seal interface topography evolvement rule in the wear process of a movable ring and a stationary ring, and a contact compression stress variation rule of primary and secondary seal interfaces due to the wear-down of the movable ring and the stationary ring and the stress relaxation of an O-shaped ring, the topography parameters of the seal interfaces and loads on the movable ring and the stationary ring seal interfaces, the O-shaped ring and the internal diameter and external diameter contact interfaces of the O-shaped ring are determined after the mechanical seal runs for a certain time, and the characteristic sizes of percolation channels between the movable ring and the stationary ring seal interfaces and the O-shaped ring and the internal diameter and external diameter contact interfaces of the O-shaped ring are obtained by a percolation theory, a contact mechanics theory and a fractal theory at the moment, the effect of surface tension and centrifugal force of the sealed medium is considered, and then the leakage state of the existing contact mechanical seal is predicated by a Navier-Stokes equation. The method is simple and practical, the accident problems caused by frequent replacement or expiration of the mechanical seal can be effectively solved, and the safety and reliability of equipment are improved.