As secure infrastructural software, secure operating systems are well known for its protection against kinds of threats and attacks. In this paper, we report the work of building EARTH operating system towards to “structurized protection” level of national security standards. EARTH has a flexible architecture supporting dynamic multiple policies, effective mandatory access control mechanism. We give formal system specification and proof of the security models and provide covert channel analysis and mitigation methods. Our experiments show that EARTH has good system performance. Furthermore, the research and development experiences and lessons learned in EARTH project is discussed.