The invention discloses a method for optimizing the horizontal arrangement distance of a hazardous chemical substance gas detector. According to the method, aiming at a target chemical engineering device, various factors including meteorological conditions, surrounding environments, device leakage risks, historical accident cases, medium types, process parameters and the like are comprehensively considered, FLACS software is used for simulating various leakage scenes, and the detector horizontal arrangement distance capable of coping with the various leakage scenes is obtained through analysis. The credibility of a set accident scene is improved, the influence of geographical area factors on hazardous chemical substance gas monitoring can be reflected more accurately, and the temperature,pressure and components of gas leaked from different accident positions of the same device can be analyzed. Besides, the method considers various accident scenes of the same set of device, and integrates analysis results, so that the arrangement position of the detector can cope with all possible accidents on the premise of ensuring timeliness.