本文以警察法第二條「防止一切危害」為出發點，先瞭解危害及與危害相似之概念，並說明危害與公共秩序和社會安全並列之不當的原因；隨即轉入正文，提出主觀與客觀、具體與抽象兩套危害概念的判斷模式，並闡述兩套判斷模式的交互為用，以修正及補強警察職權行使法和集會遊行法的相關內容；文末並以兩則案例印證之。危害尚未發生而能夠予以防止，正符合不戰而屈人之兵的最高原則，然未發生之事無法未卜先知，只能預測得知，考量警察勤務的特性，本文主張以合理預測修正懷疑強度與危害量化，合理預測可再分為表徵危害與危害疑慮兩種類型；合理預測雖植基於線索，終究是一種評估與診斷，為符合比例原則以保障人權，必須再與具體危害搭配，並依損害分量與損害逼近區分為重大危害、當前危害、顯著危害和迫切危害，然潛藏危害不與焉。危害的概念與警察職權的行使必須連結，才能兼顧安全與自由，因而回歸警職法與集遊法，提出建議，期能有所助益。

This paper is based on the phrase to prevent all hazards in Article 2 of the Police Act. First, to understand the concept of hazard and concepts that are similar to hazards, and explain the reason for improperly associating of hazards with public order and social security. It then proposes two judgment modes as subjective and objective, concrete and abstract. Furthermore, the paper attempts to revise the contents of the Police Enforcement Act and Assembly and Demonstration Act by explaining the interaction between the two modes and illustrating it with two cases. This paper advocates using a reasonable forecast to determine strength of suspicion and quantify the degree of danger. Considering the characteristics of police duty, reasonable forecast is further divided into semblance risk and suspicion of danger. For the sake of conforming to the principle of proportionality to protect human rights, we evaluate damage quantity and damage approaching significant risk, present danger, immediate imminent danger, and imminent danger. The concept of hazard must be related to the enforcement of police authority with respect to the Police Enforcement Act and Assembly and Demonstration Act.