Hoek-Brown failure criterion is one of the widely used rock strength criteria in rock mechanics and mining engineering. Based on the theoretical expression of Hoek-Brown parameter m of an intact rock, the parameter m has been modified by crack parameters for fractured rocks. In this paper, the theoretical value range and theoretical expression form of the parameter m in Hoek-Brown failure criterion were discussed. A critical crack parameter B was defined to describe the influence of the critical crack when the stress was at the peak, while a parameter b was introduced to represent the distribution of the average initial fractures. The parameter m of a fractured rock contained the influences of critical crack (B), confining pressure (σ3) and initial fractures (b). Then the triaxial test on naturally fractured limestones was conducted to verify the modification of the parameter m. From the ultrasonic test and loading test results of limestones, the parameter m can be obtained, which indicated that the confining pressure at a high level reduced the differences of m among all the specimens. The confining pressure σ3 had an exponential impact on m, while the critical crack parameter B had a negative correlation with m. Then the expression of m for a naturally fractured limestone was also proposed.