On-chip gas sensor based on infrared absorption spectroscopy has the advantages of portability, low power consumption and no need for optical path calibration. Therefore the optical gas sensor has gradually evolved from a large-size discrete system to an on-chip integrated system in recent years. The on-chip gas sensor detects gas based on waveguide evanescent field, and the interaction effect between light and gas is determined by the External Confinement Factor (ECF). However, the ECF of the waveguide is generally smaller than 1. Waveguide length is small and has a non-negligible loss. Compared with the discrete gas sensor system, these disadvantages undoubtedly reduce the sensitivity of the on-chip gas sensor. The following points need to be considered when fabricating an on-chip gas sensor: 1) Appropriate core and cladding materials should be selected to reduce absorption loss at the operating wavelength; 2) Choose and optimize waveguide structure to improve ECF; 3) Consider the feasibility and cost of the fabrication process; 4) Choose appropriate waveguide length according to waveguide loss to improve signal-to-noise ratio; 5) Choose appropriate spectroscopy sensing technique to increase sensitivity.In this review, the on-chip gas sensing method is reviewed, including Direct Absorption Spectroscopy (DAS), Wavelength Modulation Spectroscopy (WMS) and Micro-Cavity-Enhanced Absorption Spectroscopy (MCEAS). DAS technique detects the variation of light intensity, which is easily affected by noise. WMS processes signal in high frequency region and can suppress noise. MCEAS can achieve a large optical path length by an optical cavity with a compact size.Waveguide material and structure are reviewed. Gas has a larger absorption coefficient in the mid-infrared than that in the near-infrared. So transparent material is needed to decrease absorption loss. The commonly used waveguide material and platform for gas sensing are summarized. In addition, the cross-section structures of conventional waveguides are summarized, including rectangular waveguide, rib waveguide, slot waveguide, pedestal waveguide and suspended waveguide. The non-suspended waveguide has low ECF than the free-space with ECF = 1. The suspended waveguide makes full use of the area at the bottom of the core layer, so the ECF of the suspended waveguide can be > 1. But the difficulty of the fabrication process also increases. Photonic crystal waveguide has slow light effect, which can effectively improve ECF. But the loss of the photonic crystal waveguide is larger and limits the size of the interaction length.Theoretical design progress and experimental progress of on-chip gas sensors are summarized. In terms of waveguide sensor structure, sensing waveguide gradually evolved from non-suspended structure to a suspended structure to improve ECF. In the aspect of on-chip integration, on-chip sensors are gradually developing from non-integration (only waveguide) to semi-integration (integrated laser or detector) and full integration. In addition, other physical effects are also combined, such as the adsorption effect and surface-enhanced infrared absorption effect.Finally, the development direction of the on-chip gas sensor has been prospected. First, the biggest advantage of on-chip gas sensing is miniaturization, but the monolithic integration technology of light source, sensing waveguide and detector still needs to be developed, which limits the application of the on-chip gas sensor. In addition, the material and fabrication process of the sensing waveguide are also required to be compatible with the light source and detector. Second, new infrared transparent material can be used to fabricate optical waveguides to reduce the absorption loss of the material. Third, the waveguide length of an on-chip gas sensor also limits sensitivity, and other physical effects can enhance the sensitivity. Fourth, the structure of the optical waveguide directly affects the ECF, so it is necessary to reduce the fabrication difficulty as much as possible on the premise of monolithic integration and large ECF to achieve mass production. Fifth, advanced sensing technology can reduce the noise level of the sensor and improve sensitivity.