Since 2004, our lab engaged in the study of gas relaxation and ultrasonic gas detection, and in recent years, domestic research in this field  almost from our team. We use not only  the monte carlo and analog circuit but also a variety of computer simulation to study gas relaxation. The theory put forward by analytical model fits very well with the foreign theoretical model and experimental data (including CH4, N2, CO2) , and this theory laid the foundation for the study of other kind of light hydrocarbon gas with relaxation theory model . Decomposition of coupling analytical model of the solution and composite model has analyzed  relatively clearly the gas molecular vibrational relaxation, and laid the foundation for subsequent hydrogen molecular research of rotational relaxation. We have proposed  a method of wavelet transform and support vector machine (SVM) to extract gas component information spectrogram , and a smoothing algorithm of 0.01% precision as relaxation of weak gas concentration detection, that made it possible to extract information from the measured absorption hydrogen relaxation, solve the problem of natural gas pipeline in high voltage low frequency effect . Also we proposed one method to detect quaternary mixed gas composition with  sound speed, sound absorption, relaxation frequency, at the same time that laid a foundationfor for ultrasonic detection problem in super multivariate mixed gas. Moreover we put forward that without gas density but only two frequency point can reconstruct gas main relaxation process  and one gas detection method based on ultrasonic wave peaks. Above all  promotes it to be practical in monitoring the mixing of hydrogen gas components with ultrasonic method.