The present study on islanding and anti-islanding is mainly based on the characteristic of the distributed generator, however there is few research on anti-islanding by relay protection based on the electrical characteristics of the public power grid when islanding occurs. In order to find a new method which is using public power grid relay protection for anti-islanding, the analysis of the formation of islanding in Suzhou power grid is given, and the changing regulation of the zero-sequence voltage is then found. So the relay protection of public power grid for anti-islanding can be designed with zero-sequence voltage. The relay protection for anti-islanding of distributed generator is proved reasonable both by the simulation on PSCAD and the practical application of the strategy in power grid of Suzhou. The study proves that the relay protection strategy has high reliability and practical prospect.