The invention discloses a method for preparing maleic anhydride by oxidation of n-butane. According to the invention, n-butane, air and cycle tail gas are mixed, a mixture passes through more than two of reaction zones which are connected in series in order, under oxidation reaction condition, the mixture is contacted with a vanadium phosphorus oxide catalyst; besides the most downstream reaction zone, each reaction zone comprises more than two catalyst bed layers; wherein, according to a contact sequence with reaction gas, average valence of vanadium in the catalyst in the downstream reaction zone is higher than the average valence of vanadium in the catalyst in the upstream reaction zone; and, in each reaction zone besides the most downstream reaction zone, average valence of vanadium in the downstream catalyst bed layer is higher than the average valence of vanadium in the upstream catalyst bed layer. The catalyst activity is equally distributed according to reaction requirement, catalytic activity is fully performed, especially in a tail gas cycle utilization technology, influence of decreased conversion rate due to decreased oxygen content in the raw materials can be compensated, reaction hot spot is reduced, and product selectivity is increased.