Isotropic pitch-based carbon fiber was successfully prepared from tetrahydrofuran-soluble fraction of coal tar pitch cocarbonization with petrolatum by air-blowing. The effects of reaction temperature and time, amount of petrolatum added on the composition and spinning properties of resultant pitches were investigated. It indicated that petrolatum could effectively improve the softening point, aromaticity, hydrogen content and molecular weight of the resultant pitches by promoting cross-linking and dehydrogenation polymerization reactions at low air-blowing temperature. Moreover, more aliphatic and naphthenic structures had been introduced into resultant pitches as addition of petrolatum and also inhibited the generation of quinoline-insoluble particles. The obtained green fibers were facile to be stabilized and carbonized and the resultant carbon fibers showed fully isotropic and finer, uniform diameter with smooth surface and higher tensile strength of up to 0.92 GPa. It provided a facile chemical modification method for isotropic pitch-based carbon fiber production.