In the engineering applications of distributed Brillouin optical fiber sensing systems, it is often encountered that the spatial resolution of the system is greater than the length of the event zone, resulting in the acquired Brillouin gain spectrum with double peaks. When the curve fitting method and the centroid method were used to extract Brillouin frequency shift (BFS) of the double-peak gain curve, the physical quantities cannot be measured accurately in the event zone. In this paper, we propose to extract the BFS of the double-peak Brillouin gain curve using the cross-correlation method, in which the triangular curve was used as the reference curve. We study the effects of the frequency sweep steps, full width at half maxima (FWHM) of the reference curve, and signal-to-noise ratio on the BFS extraction using triangular cross-correlation. The ability of Lorentzian cross-correlation and triangular cross-correlation to extract BFS of the double-peak gain curve was compared, and the results showed that triangular cross-correlation method can extract the BFS of the double-peak Brillouin gain curve more accurately. By extracting the BFS of the double-peak Brillouin gain spectrum acquired experimentally, it is shown that the triangular cross-correlation method is more effective than the Lorentzian fitting method and the centroid method in extracting the double-peak BFS.