Because of the flexibility of hollow waveguide, the waveguide with several meters long can be bent inside a round box with a radius of only several centimeters. By using light source and detector, the miniaturized gas sensing system with long optical path, small gas volume and fast response can be obtained. A curved waveguide transmission model based on geometrical optics theory is established. The transmission and sensing characteristics of bent hollow waveguide are investigated by simulation and experiment. The methods to reduce system detection limit and improve sensitivity by optimizing such parameters like the waveguide length, bending radius, system signal to noise ratio, waveguide aperture and light source divergence angle are summarized.