In order to study the failure modes and mechanism of geotextile bag with sand cofferdam of immersed tunnel in high water level environment, the failure modes of instability of high slope rate geotextile bag with sand cofferdam of Zhoutouzui Immersed Tunnel in high water level environment are discussed by laboratory experiment and theoretical analysis. The damage control indices and evaluation methods are proposed and verified by numerical calculation. The study results show that: (1) In hard foundation, the tensile failure of stress concentration and strain localization failure will occur at the corner and interface of geotextile bag with sand cofferdam respectively under vertical load; and the internal or integral sliding will occur to geotextile bag with sand cofferdam under lateral load. (2) In soft foundation, partial detachment or tension failure will occur to cofferdam and bottom geotextile bag respectively due to insufficient foundation bearing capacity. (3) In practice, the geotextile bag are hard to be perforated and cannot form the integral curved sliding inside the cofferdam. (4) The geotextile bag with sand cofferdam has obvious package enhancement effect, which can effectively improve the overall stability; and the design of slope rate of geotextile bag with sand cofferdam in high water level environment is significantly higher than that of conventional earthrock dam cofferdam.