The depth of earth cover for tunnel anti-buoyancy security during construction and operation was analyzed in this paper with theoretical and numerical methods. The principles and theoretical analysis methods were proposed for it. The formula of depth of earth cover for anti-buoyancy security of tunnel during construction was given for the first time. It involves the unfavorable factors， such as uncondensed mortar， the favorable factors， such as resistances of soil and segments. The weight of earth cover for tunnel was suggested to consider as unique factor for tunnel security during operation. The eroded depth for design should be considered for tunnel influenced by floods. It has provided some references for longitudinal section design of the tunnel in construction under the Yangzi River in Nanjing. And it also provided theoretical foundation for the set depth of earth cover for anti-buoyancy security of other shield tunnels under river or sea.