Internal short circuits (ISCs) pose a significant safety threat to lithium-ion batteries (LIBs), and precise diagnosis relies on in-depth understandings of ISC mechanisms and characteristics. This study systematically investigated the electrochemical impedance evolution process triggered by different degrees and modes of ISCs in NCM/graphite semi-solid lithium-ion batteries. We designed a controllable introduction of ISCs with variable intrusion types and penetration depths, and based on equivalent ISC current, classified ISCs into three severity levels: soft, moderate and hard. Comprehensive assessments, including electrical-thermal responses, self-discharge rates, Coulombic efficiency (CE), incremental capacity analysis (ICA), and disassembly detection, were conducted on ISC LIBs. Using Electrochemical Impedance Spectroscopy (EIS) testing and Distribution of Relaxation Times (DRT) analysis techniques, we conducted a detailed characterization of the impedance characteristics of ISC LIBs. We found that as the degree of ISC deepens, the charge transfer impedance of LIBs decreases significantly, while the polarization resistance of the liquid phase transport process increases. However, the external short circuit (ESC) of the comparative verification does not exhibit these characteristics. The findings and analytical insights derived from this research offer a viable pathway for developing effective fault diagnosis schemes for LIB internal short circuits.