This study is aimed at shedding some light on the evolution mechanism of information service industry cluster from social network perspective. Using panel data from four typical cities in Yangtze River Delta (YRD) over the period of 2000-2011, we establish cluster networks based on the affiliation network matrices. And then we investigate the evolution of these networks by examining network characteristics at both the ego network and whole network levels. The results indicate that: 1) the average capability of an organization in getting accessing to resources is increasingly strengthened with the enlargement of network size and the enhancement of connectedness. The connectedness of each cluster network reaches as high as 100% in the growth period. 2) Growth of connections within cities far surpasses growth of connections between cities, which consequently results in the pattern with Shanghai as the leader and Nanjing, Hangzhou, and Suzhou as followers. 3) Metrics show evidence for the existence of small world structure in the cluster networks and the efficiency of information dissemination within each cluster network is quite high.