To enhance the localization capability of Brillouin optical time-domain reflectometry (BOTDR) for weak temperature variations along optical fibers, a method based on Brillouin frequency shift (BFS) fluctuation calculation is proposed. This method uses the BFS of a temperature-invariant fiber as a baseline, which is processed via sliding averaging to establish a reference benchmark. The measured BFS data under temperature variation undergoes identical processing and is then compared pointwise with the benchmark. This enables the localization of weak frequency shift points and the inference of their corresponding temperature variation ranges. Experimental results demonstrate that, without adding any additional hardware components to the system, the proposed method successfully identifies minute temperature change regions of 0.5 ℃, leading to improved system performance.