To reveal the failure mechanism and dynamic catastrophe evolution law of pillar group and solve the problem of large-area goaf collapse caused by pillar group instability in gently inclined orebody mining, the PFC2D numerical simulation method was adopted to conduct parallel uniaxial loading test simulations on three specimens with different strengths. The failure force chain and crack evolution characteristics of multiple pillars were analyzed, the chain instability mechanism of multiple pillars was revealed, and a key pillar theory based on trigger pillars, chain-breaking pillars, and critical pillars as well as a key pillar identification method were proposed. The method was verified by using the Fetr6 chromite mine case. The research shows that the instability of pillar group follows the law of chain failure, and pillars fail sequentially in the order of their strength from low to high. The trigger pillar is the weakest pillar that fails first during the chain failure process. The chain-breaking pillar can stop the propagation of chain failure in the pillar group. The critical pillar determines the critical state of overall instability of the pillar group system. The proposed key pillar identification method can accurately identify key pillars in the chain dynamic instability process. Verified by the Fetr6 chromite mine case, when the pillar bearing area loss rate reaches 0.523, the pillar group system can be judged as overall unstable. The research results provide a theoretical basis and technical support for the stability assessment and disaster prevention and control of pillar group in mines.