By means of visibility graph algorithm,the indices of monthly,weekly and daily returns of Shanghai and Shenzhen stock markets were inverstigated.It is found that the series can be converted intoa scale-free network with hierarchical structure.The relationship between the scaling exponents of the degree distributions and the Hurst exponents obeys the linear law,which proves the visibility graph can give reliable values of Hurst exponents of the series.Comparing the returns of Bull and Bear markets between 2007 and 2008,it reveals the nontrivial nature of the network topology structure of stock market.