Abstract: In order to improve the long-term sealing capacity of low-density cement sheath with hollow microsphere,the experimental studies for cement sheath integrity under the working conditions of cyclic loading and high internal pressures have been conducted,and analyzed the damages and deformation law of set cement by using the method of cyclic axial loading and unloading in tri-axial rock mechanics system. On the basis of experiment,the isolating integrity of cement sheath under high internal pressure was evaluated by thick-wall cylinder theory. Experimental results showed that both cyclic loading and high internal pressures could lead to the losses in isolating integrity of low-density cement sheath with hollow microsphere. Therefore,the dispersible fiber FK was optimized to enhance the toughness of the low-density cement sheath with hollow microsphere. The modified cementing slurry has good performances,which can meet the requirements of cementing operation. The tensile strength,coordinated deformation capacity and loading capacity of the modified set cement were all improved,while the elasticity modulus was reduced. The results indicated that dispersible fiber FK had a good effect on toughness increment,which may provide the theoretical basis and optimization method to enhance the integrity of the low-density cement sheath.