The change of flammability limit of n-heptane by the addition of halogenated hydrocarbon was studied. Experimental results showed that halogenated hydrocarbon has a combustion suppression effect and the heat capacity was a Important factor on the flammability limit. The combustion suppression effect of halogenated hydrocarbon was lower than halon, but higher than nitrogen, and the order of effect was $C_3$C1$_3$F$_3$> $C_2$HC1$_2$F$_3$> $C_2$H$_2$F$_4$.