The gas detection system composed of substrate-integrated hollow waveguide (iHWG) and Fourier transform infrared spectrometer (FTIR) has the advantages of small volume, fast response time, flexible optical design and good mechanical stability, which can be used for real-time online detection of pollution gases. The geometric parameters of iHWG, such as waveguide length, cross-section width and waveguide shape, can greatly affect the detection performance of FTIR-iHWG system. Based on the transmission theory of hollow waveguide, the influence of different geometric parameters is firstly analyzed on the transmission characteristics of iHWG. Then, four kinds of iHWG with different geometric parameters are designed using NO₂ gas with a volume ratio of 100 × 10^-6 as sample gas, and the correctness of the theoretical analysis is verified experimentally. The results show that under the same conditions, the larger the cross-section width of the waveguide, the higher the detection performance of the system. And for different shapes of iHWG, the linear iHWG system has better detection performance. The results of this work will provide useful reference for further optimization design of iHWG.