Non-Governmental Organizations, or NGOs, due to their non-state nature, always have to face the question of legitimacy of their existence in Chinese society under an authoritarian state. Whereas traditional party-state media have adopted a vigilant approach to NGOs, the booming Chinese social media have provided alternative communication platforms for NGOs in China. Based on a case study of Love Save Pneumoconiosis (LSP), a domestic originated grassroots charity NGO, this research aims to illustrate how Chinese grassroots NGOs have strategically deployed social media to manage self-organization, construct self-legitimacy, and promote advocacy goals. Both empowering potentials and self-limiting practices of social media usage in NGO activism are identified. A balanced and complementary media strategy as well as hybridized narrative strategies which depoliticize the advocated issue in general but tactfully politicize it at partial levels are adopted to ensure self-legitimacy and maximize communicative effectiveness.