As we all know, the on-chip waveguide with high Brillouin gain has a wide range of applications in the field of photonics. Brillouin lasers on silicon substrates are widely used in frequency tunable laser emission, mode-locked pulsed lasers, low-noise oscillators and optical gyroscopes. However, in silicon-based Brillouin lasers, there is still a long waveguide length to achieve Brillouin laser output, which is not conducive to on-chip integration. This paper proposes a new type of waveguide structure consisting of chalcogenide As2S3 rectangles and an air slit. Due to the existence of the air gap, the radiation pressure makes the enhancement of Brillouin nonlinearity far more than the enhancement only caused by the material nonlinearity. This makes the Brillouin gain reach 1.78×105W-1m-1, which is nearly 10 times larger than the previously reported backward SBS gain of 2.88×104W-1m-1, resulting in phonon frequency tuning in the 4.2-7GHz range. This method provides a new idea for designing nano-scale optical waveguides for forward stimulated Brillouin scattering, and at the same time, this enhanced broadband coherent phonon emission paves the way for the hybrid on-chip CMOS signal processing technology.