Driven by globalization and digitization, contemporary social public security exhibits distinct systemic and nonlinear features, rendering traditional causal-linear analytical frameworks inadequate. This paper introduces thermodynamic “entropy” to construct a “Social Public Security Entropy” framework, exploring the system’s dynamic balance and governance pathways. It elaborates entropy theory’s core connotation and its metaphor for the system, analyzes entropy increase mechanisms from endogenous and exogenous dimensions, proposes a tripartite (institutional, technological, cultural) negative entropy mechanism, and builds a 15-indicator public security evaluation system based on the PSR model. Using 2019–2023 data of a specific city and the entropy weight method, empirical tests verify the framework and negative entropy measures’ effectiveness. This study breaks disciplinary boundaries, expands entropy theory’s social science application, and provides tools for shifting public security governance from passive response to active prevention.