Abstract: The fracture process including crack initiation, stable crack propagation and unstable fracture in concrete can be described quantitatively by the double-K fracture criterion. In this paper, the three-point bending concrete beam specimens are classified in 7 groups, with the initial crack-depth ratios set from 0.3 to 0.9, respectively. The initial fracture toughness K_ICⁱⁿⁱ and unstable fracture toughness K_IC^un are calculated from the initial load P_ini, the maximum load P_max and critical crack mouth open displacement CMOD_C. The results indicate that if the initial crack-depth ratios are between 0.3 and 0.7, the concrete crack propagation process will go through crack initiation, crack propagation and unstable fracture, and the double-K fracture parameters are independent of an initial crack-depth ratio. If the initial crack-depth ratio is greater than 0.8, the concrete crack enters the stage of unstable fracture when initial cracking happens, and the initial load is equal to the maximum load, and K_ICⁱⁿⁱ is still independent of an initial crack-depth ratio. Therefore, K_ICⁱⁿⁱ can be calculated from P_ini, which equals to P_max when the initial crack-depth ratio equals to or greater than 0.8. Compared to the previous method, the results are more accurate, and the test method is simpler.