The subtropical hurricane, ocean circulation, and excavation disturbance by deep engineering of island reefs bring many challenges for the insight into the mechanical behaviors of coral reef limestone (CRL) under cyclic loads. This paper investigated the damage characteristics of coral gravel limestone (CGL) under triaxial cyclic loads based on the in-situ computed tomography (CT) technique and digital volume correlation method (DVC). The results suggested that the plastic strain of CGL still existed after unloading in the stage of linear elastic deformation. Crack initiation within the specimen occurred at the stress level of about 0.72, and accelerated damage occurred when the stress level exceeded 0.86. Subsequently, we utilized DVC technique to visualize the evolution process of 3-D strain field under cyclic loads. The strain field of the specimen generated strain release in the locality of crack initiation, and the heterogeneous degree of strain field firstly increase and then decrease with the increase of upper limit pressure. Moreover, the secant modulus exhibits a trend consistent with the heterogeneous degree of strain field. Therefore, we introduced the concept of self-information entropy to characterized the heterogeneous degree of strain field, attempting to describe the process of compression and damage of sample. The evolving relationship between secant modulus and axial strain of CGL under cyclic loads was established based on the heterogeneity of strain field. The proposed model provides promising way to predict secant modulus of porous rock materials based on-site CT testing and in-situ measurement in practical engineering.