Based on an approximate model for acoustic waves in a silica rod with circular cross section, we theoretically investigated the dispersion properties of the hybrid acoustical modes and the acousto-optical interactions in the small core photonic crystal fiber. The calculation result shows that the dual-peaked Brillouin scattering gain coefficient spectrum is caused by the mode coupling of the hybrid acoustical modes in the small fiber core. Together with the dispersion properties of the acoustical modes, the influence of the pump wavelength and temperature on the dual-peaked Brillouin scattering gain coefficient spectrum are also calculated via their influences on the acoustic wave vector and material parameters. Our theoretical results agree with the experimental results well. And we also discuss the limit of application of this model through comparing the theoretical and experimental results.