Abstract: According to the load/unload response ratio theory and its inadequacy and limitation, unload/load response ratio theory (ULRR for short) is introduced first in this paper, and the relationship between ULRR and elastic modulus E is obtained. Based on the basic theory of damage mechanics, the relationship between ULRR and damage variable D is set up and analyzed with the relationship between E and D. Using WE-100 type universal material tester, the stress-strain test of coal specimen is conducted. According to the stress-strain curve, unload/load response ratio and its change trends are calculated and analyzed in order to obtain the damage variable of coal specimen in the unloading and loading experiments. Therefore, for rock fracture experiment, it is feasible to utilize ULRR evolution laws defined by the stress-strain relationship to predict the rock damage extent.