Under the background of prosperous development of E-commerce and old city renewal, traditional wholesale markets are declining, and their transformation and upgrading are needed. In order to guide the upgrading of wholesale markets and old city space, this study took the Guangzhou Yide Seafood Market as an example to examine the mechanism and effects of its transformation. The author conducted an in-depth field research from November 2016 to March 2017 by methods of participatory observation, structured interview, snowball sampling, and questionnaire survey. Based on the history of the Yide Market from the Ming Dynasty, this article draws an preliminary conclusion that the market in recent years experienced a recessionary stage, followed by the emergence of E-commerce and comprehensive upgrading. The article then discusses the transformation mechanism and its effects by constructing an actor-network model of E-commercialization of the Yide Market. It analyzes the effects of human and non-human actors in the process of transformation by in-depth field investigation and shows that the Yide Market contains five kinds of actors—customers, the government, businesses, commodities, and network platform. In the stage of "problematization," the non-human actors, networks, and seafood dry cargo commodity showed strong guiding power and control. Then the government mobilized other actors to participate in E-commerce in the stage of "enrolment and mobilization," and businesses made different choices in the "interestement" stage. Next, role differentiations of businesses and customers became apparent, caused mainly by the actor network. Finally, the measures of self-guiding and self-management by the chamber of commerce maintained the feasibility of E-commerce along with supervision by the government. Thus the alliance of interests of the actor network was finally built. The research found that after the E-commerce transformation, traditional wholesale market would not develop to the extremity of single E-commercialization or geospatial invalidation, but form comprehensive and multiple commercial activity forms such as combinations of online and offline wholesale and retail, and that diverse powers of actors would lead the transformation of traditional wholesale markets. Non-human subjects such as products and the Internet are contributing great effects, for example, the objective characteristics of goods to be inspected hinders the elimination of geographical space, while the Internet causes people to reconsider the importance of space and upgrade the physical goods.