China advocates the spirit of the 21st-Century Maritime Silk Road (21CMSR)-"peace and cooperation, openness and inclusiveness, mutual learning and mutual benefit". The routes of the 21CMSR need to certainly passage the South China Sea (SCS) which is surrounded by China and ASEAN countries, therefore, the jointly building the 21CMSR between China and ASEAN is a geographically natural mission. The process of jointly building shall depend on a wide range of cooperations and exchanges, which is explicitly stipulated by UN Charter, the UNCLOS, the DOC as well as many other regional agreements and documents. To construct the legal mechanisms is a significant guarantee for clarifying each party's rights and obligations and promoting their cooperations and exchanges. As a consequence, China and ASEAN should jointly build some particularly crucial legal mechanisms including a code of conduct in the SCS which should be endowed the binding force, a mechanism of the regional economic and trading cooperation, a mechanism of the regional connectivity and a mechanism of the environmental protection.