To obtain the characteristic strength and damage evolution law of rocks, the 3D particle flow code (PFC3D) was used to simulate Jinping II Yantang marble rock group samples. The uniaxial and triaxial compression tests were conducted, and the initial fracture strength and damage strength were determined from the variable characteristics of microcracks morphology and quantity. When the number of microcracks reached 0.1% of the total microcracks at the peak strength, it was indicated that the initial damage occurred and the stress was the initial fracture strength. In the uniaxial compression tests, the stress corresponding to basic fracture surface was defined as damage strength based upon the shape of crack. Meanwhile, in the triaxial compression test, the stress corresponding to the non-steady phase of crack growth was defined as damage strength based upon the number of cracks. By comparing the stress features obtained from PFC3D numerical and experimental results, it was shown that the fitting function of stress curves were quite similar, which indicated that the PFC3D simulation was a reasonable approach to determine the initial fracture strength and damage strength of marble. The study on microcracks was also significant to reveal failure mechanisms of brittle rocks.