The invention provides an iron-based catalyst auxiliary as well as a preparation method and an application thereof. The auxiliary comprises the components in percentage by weight as follows: 25%-60% of molecular sieves, 1%-20% of ferric oxide and 25%-75% of N-doped carbon nano tubes, wherein nitrogen content of the carbon nano tubes is 1 wt% to 30 wt%; ferric oxide and N-doped carbon nano tubes are loaded on the molecular sieves; and Fe-N-C centers are formed between at least a part of ferric oxide and at least a part of carbon nano tubes in the iron-based catalyst auxiliary. According to the iron-based catalyst auxiliary, the carbon nano tubes and ferric oxide are highly dispersed on the molecular sieves to generate more stable active centers; ferric oxide and nitrogen are tightly combined to support walls of the carbon nano tubes stably and form Fe-N-C centers with the carbon nano tubes, so that the hydrogenation capacity of iron is improved; the carbon nano tubes have very high strength, so that activity exertion of Fe can be effectively maintained; and the N-doped carbon nano tubes facilitate reduction of acidity of the auxiliary, and oil molecules can be effectively prevented from being excessively degraded.