In order to identify the weak units in the complex goaf group, taking the goaf of an underground limestone mine as an example, through the construction of the discrete vibration mechanics model and the analysis of the related influence factors of the strain change, a damage mode identification method for weak units is innovatively proposed. The screening process is given, and its reliability is verified from two aspects: calculation examples and practical engineering application. The research results show that the screening index of natural frequency and strain mode can accurately identify the weak units in the goaf group. Both single-body and double-body unit damage will cause the natural frequency discrimination index I_M(k) of the goaf group to be greater than 0, and the value of strain modal discrimination index I_N(x) of the damaged area will be obviously convex. With the degree of damage increasing, the value of I_M(k) and the peak value ofI_N(x) peak value are getting bigger and bigger. Under only B1 damage conditions, the higher-order I_M(k) is more sensitive than the lower-order to reflect the degree of damage, and the fourth-order index recognition effect is better; Under only Z6 damage conditions, the second and fourth-order I_M(k) recognition effect is better excellent. Under B1 and Z6 damage conditions, I_M(k) is more sensitive to the changes of B1 damage degree than Z6, and the second-order I_M(k) recognition effect is better. The results have opened up a new direction for the quantitative evaluation of the safety of the underground goaf group.