The present disclosure relates to a preparation method for lowering a production cost of a high performance carbon fiber using a nanocarbon composite carbon fiber precursor fiber crosslinked by electron beam. More particularly, the present disclosure relates to a preparation method of a nanocarbon composite carbon fiber, including a nanocarbon containing step for containing nanocarbon in a structure of a carbon fiber precursor fiber, a nanocarbon composite carbon fiber precursor fiber preparation step for forming a composite of the nanocarbon and the carbon fiber precursor fiber by electron beam irradiation to enable crosslinking for improved heat resistance of the carbon fiber precursor fiber containing the nanocarbon, an oxidation⋅stabilization step for oxidizing⋅stabilizing the nanocarbon composite carbon fiber precursor fiber, and a carbonization step for carbonizing the oxidized⋅stabilized nanocarbon composite carbon fiber precursor fiber, and a nanocarbon composite carbon fiber prepared by the preparation method.