Accessibility is one of the indicators to measure the efficiency of network.The purpose here is to analyze the rail transit network structure,and the networks of Shanghai subway transportation during three different periods were chosen for accessibility analysis.The empirical analysis results show that the accessibility is declined with the increasing of subway network scale.In order to explain the reasons,the network structure of each period was analyzed.It turns out that the degree distribution of subway transportation networks play a greater role than the average distance and average path length do.After comparing with the random networks which have the same degree distribution,there's still a large space which can be improved in the convenience and efficiency of the network.