The invention discloses a rhodium-based catalyst for preparing ethanol by a synthesis gas one-step method and a preparation method thereof; the catalyst comprises an active component, an auxiliary agent and a carrier, the active component is Rh, the auxiliary agent M is selected from one or more of alkali metal, alkaline earth metal or transition metal elements, and the carrier is cerium dioxide or a cerium zirconium solid solution; the preparation method comprises that a salt solution of the active component and the auxiliary agent is immersed in cerium dioxide or the cerium zirconium solid solution, the catalyst is obtained through drying, roasting and reduction, the active component and the auxiliary agent are evenly dispersed on the carrier, the strong interaction is formed between the active component and the auxiliary agent, and the catalyst has high activity and stability; the prepared rhodium-based catalyst is suitable for ethanol preparation process from synthesis gas, can replace a catalyst for preparing ethanol by a synthesis gas multi-step method, and has relatively high selectivity on alcohols in products; the catalyst can simplify the process route of preparing ethanol from synthesis gas to a great extent and overcomes the dependence on food and petroleum resources in a traditional ethanol synthesis route.