The mechanism of Tc suppression induced by Zn substitution for Cu in YBa2Cu3O7-y system has been studied with respect to its electronic transport properties in normal state, superconducting critical temperature, crystal structure, oxygen stoichiometry and so forth. The results show that Zn ion acts as impurity scattering centre, leading to the increase in the resistivity as well as to the anomalous changes of the resistance-temperature relation. In addition , Zn substitution for Cu reduces the carrier concentration due to the reduction of the number of Cu other than the oxidation state in CuO planes, which precludes the band-filling and carrier-over-saturation models for explaining the mechanism of Tc suppression. Semi-quantitative results reveal that each Zn ion present a scattering cross section with a radius larger than the average Cu-O bond length of the CuO planes. This means that a non-superconducting core may appear around each Zn ion. As a result, the superconducting path in CuO plane will be destroyed gradually.as a percolation process. The percolation threshold observed experimentally is consistent with the theoretical calculation. We believe that the emergence of the normal core together with the consequent percolation is the reason why Tc is suppressed so rapidly by Zn substitution for Cu.