Brillouin gain spectra (BGS) in an As₂Se₃ photonic crystal fiber (PCF) are investigated numerically. The profiles of the BGS are simulated by calculating the characteristics of different-order optical and acoustic waves in the PCFs with different core diameters. For the small-core PCF, there are two peaks in BGS, but there is only one peak for the large-core PCF. We also reveal that in the small-core PCF, the difference of Brillouin frequency shift between the LP₀₁ and LP₁₁ modes is obvious, while it is not obvious in the large-core PCF. The Brillouin threshold increases with the core diameter increasing.