The invention relates to a catalyst for preparing acetaldehyde by ethane selective oxidation and a preparation method thereof. The catalyst for preparing the acetaldehyde by ethane selective oxidation is a doped catalyst using an SBA-15 molecular sieve as a carrier, using an oxide of transition molybdenum metal as an active ingredient and using an oxide of alkali metal potassium as an auxiliary agent, wherein a part of molybdenum oxide is supported on the surface of the SBA-15 molecular sieve, and the other part of molybdenum oxide is doped into the skeleton of the SBA-15 molecular sieve; thepotassium oxide is supported on the surface of the SBA-15 molecular sieve; and the raw material mole ratio of K, Mo to Si is 0.5:10: 100. The invention also provides the preparation method for the catalyst. The catalyst, the molybdenum of which is doped into the SBA-15 molecular sieve carrier skeleton and the potassium auxiliary agent of which is supported on the surface of the carrier in a high dispersion form, has high reaction activity and selectivity; and when the catalyst is used for preparing the acetaldehyde by ethane selective oxidation, the yield of the acetaldehyde can reach 10.2 percent.