We investigate the influence of the structural parameters of the total internal reflection photonic crystal fiber (TIR-PCF) on the Brillouin gain spectrum (BGS) characteristics, including the Brillouin gain, number of Brillouin peaks, and peak relative power. Further, the finite element analysis method is used to obtain the optical and acoustic field distributions for analyzing the acousto-optic coupling effect in the TIF-PCF; thus, the corresponding BGS of the PCF can be obtained. In addition, we discuss the effects of the PCF parameters, such as the air-hole layer number, pitch, and diameter, on the BGS. The law of the Brillouin gain and the acoustic mode number is obtained as a function of the air-hole pitch and diameter. Accordingly, we propose a novel PCF structure, whose air-hole diameters gradually increase from the inside to the outside and which exhibits a similar gradient refractive index distribution. A PCF with two peaks in its BGS and a peak power difference of 8 dB is designed and expected to be used in the fiber optic sensing systems based on the Brillouin beat spectrum to increase the signal-to-noise ratio of the sensing system by a factor of 2.5.