The Chinese government announced in 2006 its commitment to build ‘a strong social work force’ to deal with the upsurge of social problems and realise the political vision of maintaining a ‘harmonious socialist society’. Assigned to perform the welfare role historically played by government cadres, volunteers or kinship networks, social workers in China have encountered many difficulties in defining and claiming their practice mandate at work sites. Based on a qualitative study conducted in Shenzhen and Guangzhou, this paper discusses the struggles social workers had, as they attempted to establish their exclusive purview of work and professional role, and uncovers the street-level micro politics in the welfare work sites, as different actors competed to define the social work mandate in their respective interests.