Based on GRIMech 3.0 mechanism, using kinetic model, the auto ignition characteristics of nature gashydrogenair mixture are studied in different initial conditions under constant pressure. This study focuses on the changes of temperature and component concentrations，ignition delay time and flame propagation speed. The results show that under different initial conditions, with the increase of hydrogen fraction in the mixture, the ignition delay time decreases significantly and flame propagation speed increases significantly. On the other hand, Hydrogen fraction has been found of little influence on flame temperature.