The indoor fire of leaked gaseous fuel was numerical studied by using FLUENT software. A partially premixed combustion turbulent model was applied for the simulation. The results showed that the flame spreads faster in the flammable zone (a thin layer horizontal to the lower edge of the outlet) than other zones. The influence of the leak time and igniting position to fire spreading were studied It was found that the longer the leak time was, the lower the combustion intensity was, and the longer the fire would last. And the combustion intensity and the fire lasting time changed with the igniting position. Experiments were done to verify the results of the simulation, and the results of experiments tended to agree well with the simulation.