Abstract: In 2002, the Greater London Authority promulgated Connecting with London’s Nature—The Mayor’s Biodiversity Strategy. In the past 20 years, through integrating with spatial plans, it has carried out biodiversity conservation strategy centered on the blue and green space management to promote the realization of diverse social and ecological values. Using policy analysis and historical research, this research examines the policies at the levels of the United Nations, the EU, the UK and London, to uncover the evolution of core ideas of policy design. London has legislated biodiversity conservation as a duty of the major, and established a comprehensive policy framework that combines habitat protection and new construction to give full play to the social and ecological value of blue and green space in environment education, green economy, public recreation and so on. By incorporating biodiversity conservation into the spatial planning, it has guaranteed long-term policy implementation and improvement. The urban biodiversity conservation strategies of London are realized by developing the sites of importance for nature conservation (SINC) system and promoting biodiversity compensation in urban development. By sorting out the evolution of the protection concept behind the multi-level policies, it illustrates the key strategies to achieve biodiversity net gain in London’s management of the blue and green space. It can provide a reference for the formulation of urban biodiversity conservation policies in the context of future territorial space planning and management.