Chinese city governments have collaborated increasingly to address regional environmental issues by participating in informal and formal collaborative networks. However, collaboration among cities involves collaboration risks. This study investigates how cities strategically select collaborative partners in informal and formal networks in the context of the Yangtze River Delta in China. This study addresses this question by assessing the nature of collaborative problems in the informal and formal networks, the extent of homophily in actors' preferences, and their relationship multiplexity. Findings from Exponential Random Graph Analysis demonstrate: (1) city governments tend to connect to the popular actor and create relationship closure in the informal network, while only forging relationship closure in the formal network; (2) homophily (in water pollution) and heterogeneity (in air pollution) jointly affect city governments' choices of collaborative partners in the formal network; and (3) the presence of relationship multiplexity wherein the formation of formal ties is built between city governments with pre-existing informal interactions. The findings advance our knowledge of collaborative partner selection and local collaborative governance in China.