A process for preparing large-quantity nm-class carbon fibres by using sulfur growth promoter features that benzene, CH4 or C2H2 is used as carbon source, the ferrocene or Ni(Co)4 as catalyst, the hydrogen, helium, or nitrogen as carrier gas, the sulfur-contained compound (thiophene or H2S) as growth promoter, and the mole ratio is 0.05-0.25 for carbon to carrier gas, or (600-1300) : 1 for carbonsource to sulfur, or 0.008-0.015 for catalyst to carbon source. The carbon fibre is prepared through fully mixing sulfur-contained compound, hydrocarbon and catalyst in gas state, carrying the mixture in reaction region by carrier gas at uniform speed, and reacting at 1373-1473 deg.K for 0.075-0.5 seconds. Its advantages are high output rate and low cost.