Taking Tianxingzhou coast protecting project implemented at the Zhoutian reach of middle reaches of the Yangtze River,the paper describes the shrinkage,tearing and breakage phenomenon of type D flexible mattress in mattress sinking construction under complex flow condition,analyzes uncertain factors of the shrinkage,tearing and breakage phenomenon,and probes into the mechanism of the above-said phenomena from the angle of force loaded.The result provides a guidance for the design of flexible mattress and mattress sinking construction under complex flow condition.