Social entrepreneurship has gradually become an important force to break through market and government failures, and promote social changes by the virtue of its innovative approach to solve social problems. Many social start-ups have been built under the circumstance of the entrepreneurial craze, but the number of start-ups that can achieve steady growth in reality is not satisfactory. The growth of social start-ups faces many problems such as serious resource constraints and sustainable development. Social start-ups often reject the optimal resource allocation in response to resource scarcity and take the resource bricolage model to support business development. However, due to the dynamic nature of social entrepreneurial growth, there may be a complex mechanism between resource bricolage and the growth of social start-ups. Therefore, this study integrates the perspectives of resource bricolage and organizational legitimacy to explore social start-ups’ growth mechanism model by in-depth case studies with four social enterprise cases. Research results show that different types of resource bricolage can help social start-ups obtain different organizational legitimacy. Physical bricolage, skill bricolage and market bricolage mainly help to obtain market legitimacy to promote the economic growth of social start-ups, while labor bricolage and institutional bricolage help to obtain social legitimacy to realize the social value of social start-ups. Meanwhile, different types of social start-ups are different in patterns of legitimacy and degrees of growth at different stages of growth. The above conclusions have some significance. In theoretical aspect, firstly, this study reveals the process mechanism of social start-ups at different stages of growth through case studies, and provides a new research path for enriching and expanding social entrepreneurship research. Secondly, this study embeds the resource bricolage theory into the field of social entrepreneurship, clarifies the mechanism that resource bricolage promotes the growth of social start-ups, and expands resource bricolage theory research. Finally, this study echoes the calling by Wilson et al.（2013）for social entrepreneurship legitimacy to break through the zero-sum game dilemma of the economic and social value, enriching and supplementing the research results in the field of organizational legitimacy. In practice, firstly, this study inspires social entrepreneurs to establish bricolage thinking to ease the resource dilemma through the creative use of resources at hand; at the same time, we should attach importance to and strengthen the role of resource bricolage in the process of obtaining legality, and apply limited resources to the " blade”, in order to " fit for the time”. Secondly, social entrepreneurs should pay attention to the thinking of the entire enterprise structure and strategy, and understand the interaction between the enterprise and the environment. We need to maintain a relative balance between market legitimacy and social legitimacy through a combination of different resource bricolage models.